Pilot stabilized burner

ABSTRACT

According to an embodiment, a burner system includes a pilot burner disposed in a furnace at a distal position along a main fuel and combustion air flow axis, and one or more main fuel nozzles disposed at a proximal position along the main fuel and combustion air flow axis. The pilot burner is configured to support a pilot flame and the one or more main fuel nozzles are configured to support a main flame in contact with the pilot flame. The pilot burner is disposed to cause the main fuel and combustion air to be ignited by the pilot flame. The pilot burner may support a diffusion pilot flame or may include a premixing apparatus to support a pre-mixed flame.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. Continuation-in-Part application which claims priority benefit of co-pending International Patent Application No. PCT/US2020/031966, entitled “PILOT STABILIZED BURNER”, filed May 7, 2020 (docket number 2651-348-04). Co-pending International Application PCT/US2020/031966 claims priority benefit to U.S. Provisional Patent Application No. 62/844,669, entitled “PILOT STABILIZED BURNER,” filed May 7, 2019 (docket number 2651-348-02). International Patent Application No. PCT/US2020/031966 also claims priority benefit to co-pending U.S. patent application Ser. No. 16/782,861, entitled “LOW EMISSION MODULAR FLARE STACK,” filed Feb. 5, 2020 (docket number 2651-257-05). The present application is a U.S. Continuation-in-Part application which also claims priority benefit to U.S. Provisional Patent Application No. 63/160,682, entitled “BURNER SYSTEM WITH PRE-MIXED DISTAL PILOT,” filed Mar. 12, 2021 (docket number 2651-354-02). Each of the foregoing applications, to the extent not inconsistent with the disclosure herein, is incorporated by reference.

SUMMARY

According to an embodiment, a burner system includes a pilot burner disposed in a furnace at a distal position along a main fuel and combustion air flow axis, and one or more main fuel nozzles disposed at a proximal position along the main fuel and combustion air flow axis. The pilot burner is configured to support a pilot flame and the one or more main fuel nozzles are configured to support a main flame in contact with the pilot flame. The pilot burner is disposed to cause the main fuel and combustion air to be ignited by the pilot flame.

According to an embodiment, a burner system includes a main fuel source disposed at a proximal position along a flow axis of a furnace, a pilot burner disposed at an intermediate distance along the flow axis, and a distal flame holder disposed at a distal position along the flow axis. The pilot burner is configured to support a pilot flame to heat the distal flame holder. The main fuel source is configured to provide main fuel to the distal flame holder after the distal flame holder is at least partially heated. The distal flame holder is configured to hold at least a portion of a combustion reaction supported by the main fuel.

According to an embodiment, a method for operating a burner system includes providing heat to the distal flame holder from a pilot flame supported by a pilot burner, the pilot flame being fueled by a pilot fuel. The distal flame holder and the pilot burner are disposed in a furnace and in proximity to one another, the pilot burner disposed between the distal flame holder and one or more main fuel nozzles with a distance between the pilot burner and the distal flame holder being smaller than a distance between the pilot burner and the one or more main fuel nozzles. The method for operating a burner system includes introducing mixed fuel and air to the distal flame holder and holding at least a portion of a combustion reaction of the mixed main fuel and air with the distal flame holder while the pilot burner continues to support the pilot flame.

According to an embodiment, a method for operating a burner system includes supporting a diffusion flame across a portion of a width of a furnace volume at a position distal from a furnace floor, providing combustion air to the furnace volume from a location near the furnace floor, outputting a high pressure main fuel jet from each of one or more main fuel nozzles at one or more locations near the furnace floor, mixing the main fuel with the combustion air while the main fuel and combustion air travel from the locations near the furnace floor to the distal position, combusting the main fuel to produce a main flame by exposing the mixed main fuel and air to the diffusion flame. According to an embodiment, the main flame is held by a distal flame holder more distal from the furnace floor than the diffusion flame.

According to an embodiment, a combustion system includes an oxidant source configured to output an oxidant into a furnace volume, a pilot burner configured to support a pilot flame by outputting a pilot fuel to support a pilot diffusion flame at least during a preheating state, and a main fuel nozzle configured to output a main fuel into the furnace volume from a proximal position during a standard operating state at least after the preheating state is complete. The combustion system includes a distal flame holder positioned in the furnace volume to be preheated by the pilot flame during the preheating state and to hold a combustion reaction of the main fuel and oxidant adjacent to the distal flame holder during the standard operating state. The combustion system includes a combustion sensor configured to sense a condition of the combustion system and to generate a sensor signal indicative of the condition of the combustion system, and one or more actuators configured to adjust a flow of the main fuel from the main fuel nozzle, to adjust a flow of the pilot fuel to the pilot burner, and to adjust a flow of the oxidant from the oxidant source. The combustion system includes a controller communicatively coupled to the actuators and the combustion sensor, the controller being configured to receive the sensor signals from the combustion sensor and to control the actuators to adjust the flow of the pilot fuel, the main fuel, and the oxidant responsive to the sensor signals and in accordance with software instructions stored in a non-transitory computer readable medium coupled to the controller.

According to an embodiment, a computing system implemented method for operating a combustion system includes receiving, during a preheating state of a combustion system, sensor signals from a pilot flame sensor indicating a condition of a pilot flame in a furnace volume supported by a flow of pilot fuel and an oxidant, and receiving, during the preheating state, sensor signals from a distal flame holder sensor indicating a temperature of a distal flame holder positioned in the furnace volume to be preheated to an operating temperature by the pilot flame during the preheating state. The method includes outputting control signals to control one or more actuators to adjust the flow of the pilot fuel, to adjust the flow of the oxidant, or to generate an arc to ignite the pilot flame responsive to the sensor signals from the pilot flame sensor and in accordance with software instructions stored on a non-transitory computer readable medium, and outputting control signals to control one or more actuators to transition the combustion system from the preheating state to a standard operating state if the sensor signals from the distal flame holder sensor indicate that the distal flame holder has reached the operating temperature, the standard operating state corresponding to supporting a combustion reaction of a main fuel and the oxidant in the distal flame holder and in accordance with the software instructions stored on the non-transitory computer readable medium. The method includes receiving sensor signals from the distal flame holder sensor during the standard operating state indicating a condition of the distal flame holder, and outputting control signals to control one or more actuators to adjust a flow of the main fuel or to adjust the flow of the oxidant responsive to the sensor signals from the distal flame holder sensor during the standard operating state and in accordance with the software instructions stored on the non-transitory computer readable medium.

According to an embodiment, a low emissions modular burner system includes one or more burner modules. Each burner module includes a main fuel source, separately valved from all other fuel sources, configured to selectively deliver a main fuel stream for dilution by a flow of combustion air, a main fuel igniter configured to cause ignition of the main fuel stream emitted from the main fuel source, a distal flame holder, separated from the main fuel source and the main fuel igniter by respective non-zero distances, the distal flame holder being configured to hold a combustion reaction supported by the main fuel stream when the distal flame holder is at or above a predetermined temperature, and a pre-heating apparatus configured to pre-heat the distal flame holder to the predetermined temperature. The low emissions modular burner system includes a common combustion air source configured to provide combustion air to each of the plurality of burner modules, and a wall encircling all of the one or more burner modules, the wall being configured to laterally contain combustion fluids corresponding to the one or more burner modules.

According to an embodiment, a burner includes a housing having a combustion air inlet at a base, and a burner module positioned inside the housing. The burner module includes an inlet configured to be coupled to a main fuel supply and to receive combustion air via the housing, a distal flame holder positioned inside the housing, and a main nozzle configured to receive a flow of main fuel from the inlet, and to emit a main fuel stream toward the distal flame holder.

According to an embodiment, a burner system includes a distal flame holder configured to hold a combustion reaction of a fuel and an oxidant, an oxidant conduit configured to direct the oxidant toward the distal flame holder, a main fuel nozzle oriented to direct a flow of a main fuel into a combustion volume for mixture with the oxidant in a dilution region between the main fuel nozzle and the distal flame holder when a temperature of the distal flame holder is above a predetermined temperature, and a mixing tube disposed in the dilution region, and being open from a mixing tube inlet to a mixing tube outlet between the main fuel nozzle and the distal flame holder, the mixing tube being formed to cause flow of the oxidant and fuel to reduce flue gas into the mixing tube for mixing with fuel and oxidant.

According to an embodiment, a burner system includes a pilot burner and one or more fuel nozzles. The pilot burner is disposed in a furnace at a distal position adjacent to a main fuel and combustion air flow axis. The one or more main fuel nozzles are disposed at a proximal position along the main fuel and combustion air flow axis. The pilot burner is configured to support a pilot flame using a pre-mixture of a pilot fuel and an oxidant. The one or more main fuel nozzles are configured to output a main fuel to flow from the proximal position to the distal position along the main fuel and combustion air flow axis. The pilot flame is aligned to initiate ignition of the main fuel and the combustion air where the main fuel and combustion air reach an intended distal flame front position, and the pilot burner is disposed to support the pilot flame at the intended distal flame front position.

According to an embodiment, a burner system includes a main fuel source, a pilot burner, and a distal flame holder. The main fuel source is disposed at a proximal position along a direction of a flow axis of a furnace. The pilot burner is disposed at an intermediate distance along the direction of the flow axis, and the distal flame holder is disposed at a distal position along the direction of the flow axis. The pilot burner is configured to provide a pre-mixture of a pilot fuel and an oxidant to support a pilot flame to heat the distal flame holder. The main fuel source is configured to provide main fuel to the distal flame holder after the distal flame holder is at least partially heated, and the distal flame holder is configured to hold at least a portion of a combustion reaction supported by the main fuel.

According to an embodiment, a method for operating a burner system includes providing a pre-mixture of pilot fuel and oxidant to a pilot burner, maintaining a pilot flame at the pilot burner, and igniting a flow of main fuel and combustion air with the pilot flame at an intended position distal from one or more main fuel nozzles and a combustion air source.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of a burner system, according to an embodiment.

FIG. 1B is a block diagram of a burner system including a distal flame holder, according to an embodiment.

FIG. 2A is an illustration of a burner system, according to an embodiment.

FIG. 2B is an illustration of a burner system including a distal flame holder, according to an embodiment.

FIG. 3 is a perspective view of a combustion system, according to an embodiment.

FIG. 4 is an illustration of a pilot burner in the shape of an H, according to an embodiment.

FIG. 5 is an illustration of a pilot burner in the shape of a spiral, according to an embodiment.

FIG. 6 is an illustration of a pilot burner in the shape of a hexagon, according to an embodiment.

FIG. 7 is a simplified diagram of a combustion system including a distal flame holder configured to hold a combustion reaction wherein the distal flame holder includes a perforated flame holder, according to an embodiment.

FIG. 8 is a side sectional diagram of a portion of the perforated flame holder of FIG. 7, according to an embodiment.

FIG. 9 is a flow chart showing a method for operating a burner system including the distal flame holder shown and described herein, according to an embodiment.

FIG. 10A is a simplified perspective view of a combustion system, including a reticulated ceramic perforated flame holder configured to hold a combustion reaction, according to an embodiment.

FIG. 10B is a simplified side sectional diagram of a portion of the reticulated ceramic perforated flame holder of FIG. 10A, according to an embodiment.

FIG. 11 is a flow chart showing a method for operating a burner system, according to an embodiment.

FIG. 12 is a flow chart showing a method for operating a burner system, according to an embodiment.

FIG. 13A is a diagram of a combustion system including a distal flame holder and an electrocapacitive combustion sensor, according to an embodiment.

FIG. 13B is a top view of the distal flame holder and an electrocapacitive combustion sensor, according to an embodiment.

FIG. 14 is a diagram of an arrangement for each of a plurality of burner modules, according to an embodiment.

FIG. 15 is a diagram of a control circuit for use in the control system of FIG. 14, according to an embodiment.

FIG. 16 is a block diagram of a burner system, according to an embodiment.

FIG. 17 is an illustration of a burner system, according to an embodiment.

FIG. 18 is an illustration showing a horizontally-fired burner system including a distal pilot burner and a mixing tube, according to an embodiment.

FIG. 19 is a diagram of burner system including a distal pilot burner, according to an embodiment.

FIG. 20 is a flow chart showing a method associated with operating a burner system, according to an embodiment.

FIG. 21A is a flow chart showing a method associated with operating a burner system, according to an embodiment.

FIG. 21B is a flow chart showing a method associated with operating a burner system, according to an embodiment.

FIG. 22 is a flow chart showing a method associated with operating a burner system, according to an embodiment.

FIG. 23A is a flow chart showing a method associated with operating a burner system, according to an embodiment.

FIG. 23B is a flow chart showing associated with operating a burner system, according to an embodiment.

FIG. 24 is a diagram showing a combustion system, according to an embodiment.

FIG. 25 is a diagram showing a distal flame holder, according to an embodiment.

FIG. 26 is a diagram showing distal flame holder, according to another embodiment.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. Other embodiments may be used and/or other changes may be made without departing from the spirit or scope of the disclosure.

FIG. 1A is a block diagram of a burner system 100, according to an embodiment. The burner system 100 includes a pilot burner 104 and one or more main fuel nozzles 106. The pilot burner 104 and the main fuel nozzle(s) 106 are disposed in a furnace volume 101.

According to an embodiment, the pilot burner 104 is disposed in a furnace 101 at a distal position along a main fuel and combustion air flow axis A. The one or more main fuel nozzles 106 are disposed at a proximal position along the main fuel and combustion air flow axis A.

According to an embodiment, the pilot burner 104 is configured to support a pilot flame 108. As is described in more detail below, the pilot flame 108 helps to ignite and/or sustain a main combustion reaction 110.

In one embodiment, the pilot burner 104 supports the pilot flame 108 by outputting a pilot fuel 112. The pilot flame 108 is supported by the pilot fuel 112 and combustion air introduced into the furnace volume 101. Accordingly, the pilot flame 108 is a combustion reaction of the pilot fuel 112 and combustion air.

In one embodiment, the one or more main fuel nozzles 106 are configured to support the main flame 110 within the furnace volume 101. The main flame 110 is supported downstream from the pilot flame 108.

In one embodiment, the main fuel nozzles 106 support the main flame 110 by outputting main fuel 114 into the furnace volume 101. The main flame 110 is supported by the main fuel 114 and combustion air introduced into the furnace volume 101. Accordingly, the main flame 110 is a combustion reaction of the main fuel 114 and the combustion air.

In one embodiment, the pilot burner 104 and the one or more main fuel nozzles 106 are configured to simultaneously support the main flame 110 in contact with the pilot flame 108. The pilot burner 104 is disposed to cause the main fuel and combustion air to be ignited by the pilot flame 108.

In one embodiment, the main flame 110 may not be in contact with the pilot flame 108. Instead, the main flame 110 may be separated from the pilot flame 108 away by a gap.

In one embodiment, the pilot burner 104 includes a plurality of tubes or arms that extend laterally from the axis A. The tubes or arms include a plurality of orifices that output the pilot fuel. Accordingly, the pilot flame 108 is held above each of the tubes, arms, or segments of the pilot burner 104. The shape of the pilot burner 104 can be selected to cover a lateral area corresponding to the area above the main fuel nozzles 106, or an area through which the main fuel 114 passes.

In one embodiment, the main fuel 114 passes through gaps or open spaces between the laterally extending portions of the pilot burner 104. The main fuel 114 may be initially ignited by the pilot flame 108 as the main fuel 114 passes adjacent to the pilot flame 108. After the main fuel 114 has been ignited, thereby generating the main flame 110, the main flame 110 can be supported in a steady state by the main fuel 114.

In one embodiment, the laterally extending arms of the pilot burner 104 form a star shape. Additionally, or alternatively, the pilot burner 104 can form a spiral shape, a circle shape, an H shape, a square shape, a hexagon shape, or other shapes that cover a desired lateral distance while including gaps through which the main fuel 114 can pass. (See, e.g., FIGS. 2A-6.)

In one embodiment, the pilot burner 104 includes a pilot fuel manifold. The pilot fuel manifold includes laterally extending tubes, segments, arms, or portions. The pilot fuel 112 is output from orifices positioned in the laterally extending tubes, segments, arms, or portions of the pilot fuel manifold.

According to an embodiment, the main flame 110 includes a flame having a heat output of at least 10 times the heat output of the pilot flame 108 when the burner system 100 is operating at a rated heat output. In one embodiment, operating at a rated heat output corresponds to operating in a steady state standard operating mode of the burner system 100. In another embodiment, the main flame 110 includes a flame having a heat output of at least 20 times the heat output of the pilot flame 108 when the burner system 100 is operating at a rated heat output.

According to an embodiment, the burner system 100 has a NOx output of about twenty parts per million or less, adjusted to 3% excess O₂ at a stack operatively coupled to the burner system 100. In one embodiment, the burner system 100 has a NOx output of about twenty parts per million or less, adjusted to 3% excess O₂ at an exhaust stack operatively coupled to the burner system 100.

FIG. 1B is a block diagram of a burner system 111 including a distal flame holder 102, according to an embodiment. The burner system 111 of FIG. 1B is substantially similar to the burner system 100 of FIG. 1A, except that the burner system 111 includes the distal flame holder 102.

According to an embodiment, the burner system 111 includes a main fuel source 106 disposed at a proximal position along a flow axis A of a furnace volume 101, a pilot burner 104 disposed at an intermediate distance along the flow axis A, and a distal flame holder 102 disposed at a distal position along the flow axis A. The pilot burner 104 may be configured to support a pilot flame 108 to heat the distal flame holder 102. The main fuel source 106 may be configured to provide main fuel 114 to the distal flame holder 102 after the distal flame holder 102 is at least partially heated. The distal flame holder 102 may be configured to hold at least a portion of the main flame 110 supported by the main fuel 114.

In one embodiment, the distal flame holder 102 is a perforated flame holder. The operation and structure of the perforated flame holder are described with more particularity in relation to FIGS. 7-10B. In other embodiments, the distal flame holder 102 may include one or more solid bluff body flame holders or may include a mixture of one or more perforated flame holders and one or more bluff body flame holders.

FIG. 2A is an illustration of a burner system 200, according to an embodiment. The burner system 200 includes a pilot burner 104 and main fuel nozzles 106. Though not shown in FIG. 2A, the pilot burner 104 is support the pilot flame 108 by outputting the pilot fuel 112. Though not shown in FIG. 2A, the main fuel nozzles 106 are configured to support the main flame 110 by outputting the main fuel 114.

According to an embodiment, the pilot burner 104 is supported by and receives fuel via a fuel pipe 220. The fuel pipe 220 extends into the furnace volume 101 via an opening 240 in a floor 238 of the furnace. A stiffener 222 can be positioned around the fuel pipe 220 to prevent the fuel pipe 220 from wobbling. The main fuel nozzles 106 also extend through the opening 240 in the floor 238. The main fuel nozzles 106 can be supported by fuel risers 224. In one embodiment, the main fuel nozzles 106 include orifices that output the main fuel 114 with a 2° spread.

According to an embodiment, the pilot burner 104 defines a plurality of fuel orifices 218 having a sufficiently large collective area to collectively support a low momentum pilot flame 108. In an embodiment, the main fuel 114, output by the main fuel nozzles 106 and combustion air form a combustible mixture that expands in breadth as it flows from the proximal position of the main fuel nozzles 106 to the distal position of the pilot burner 104. The plurality of fuel orifices 218 may be disposed across the furnace volume 101 sufficiently broadly to cause contact of the pilot flame 108 with the main fuel 114 and combustion air mixture across the breadth of the combustible mixture. In another embodiment, the main fuel nozzles 106 are configured to output fuel in co-flow with the air.

According to an embodiment, the pilot burner 104 includes a fuel manifold having a plurality of segments 219 joined together, each segment 219 having a plurality of fuel orifices 218 configured to pass fuel from inside the fuel manifold to the furnace volume 101. The plurality of segments 219 may be formed as respective tubes configured to freely pass the fuel delivered from the fuel pipe 220 into the fuel manifold. In one embodiment, at least a portion of the tubes is arranged as spokes radiating from a center disposed substantially at a centerline along the axis. In other embodiments, at least a portion of the tubes may be arranged as an “X”, a rectangle, an “H”, a wagon wheel, or a star.

According to an embodiment, the pilot burner 104 includes a manifold including a curvilinear tube. In an embodiment, the curvilinear tube is arranged as a spiral, “

”, “

”, or “∞”.

According to an embodiment, the main fuel nozzles 106 form a main fuel dump plane at the proximal location coincident with or near the floor 238 of the furnace.

According to an embodiment, the pilot burner 104 supports a diffusion flame at the distal location at least 100 main fuel nozzle 106 diameters from the floor 238 of the furnace.

According to an embodiment, the pilot burner 104 includes at least one tube disposed transverse to the fuel and combustion air flow axis A. The at least one tube may include opposed vertical tabs extending upward from the at least one tube to form a “U” channel.

According to an embodiment, the burner system 200 includes a pilot fuel source 230. The pilot fuel source 230 supplies the pilot fuel 112 into the fuel pipe 220. The pilot fuel 112 is output from the pilot burner 104 via the fuel orifices 218. A pilot fuel control valve 234 can be manually or electronically controlled to enable or shut off the flow of pilot fuel 112 from the pilot fuel source 230 to the pilot burner 104.

According to an embodiment, the burner system 200 includes a main fuel source 232. The main fuel source 232 supplies the main fuel 114 to the main fuel nozzles 106. The main fuel 114 can be supplied to the main fuel nozzles 106 via the fuel risers 224. A main fuel control valve 236 can be manually or electronically controlled to enable or shut off the flow of the main fuel 114 from the main fuel source 232 to the main fuel nozzles 106.

According to an embodiment, the main fuel source 232 and the pilot fuel source 230 may be a single fuel source or reservoir, fuel of which may be respectively directed to the pilot burner 104 and the main fuel nozzle(s) 106 and separately controllable via, e.g., the pilot fuel control valve 234 and the main fuel control valve 236.

In one embodiment, the combustion air is provided into the furnace volume 101 as natural draft flow through the opening 240 in the floor 238 of the furnace. Additionally, or alternatively, the combustion air can be provided into the furnace volume 101 in ways other than through the opening 240 in the floor 238. For example, combustion air may include recirculated flue gas(es), as discussed in more detail below. In another example, the combustion air may include forced draft from a blower (not shown).

FIG. 2B is an illustration of a burner system 211 including a distal flame holder 102, according to an embodiment. The burner system 211 is substantially similar to the burner system 200 of FIG. 2A, except that the burner system 211 includes a distal flame holder 102 positioned above the pilot burner 104, i.e., disposed at a position further distal from the main fuel nozzles 106 than the pilot burner 104. While the pilot burner supports the pilot flame 108 (see FIG. 1A and FIG. 1B), the distal flame holder 102 holds the main flame 110 (see FIG. 1B). The pilot flame 108 can ignite and stabilize the main flame 110.

According to an embodiment, the distal flame holder 102 may include a perforated flame holder configured to hold the secondary flame 110 and to control the length of the secondary flame 110. Such perforated flame holder 102 can hold at least a portion of the secondary flame 110 within the perforated flame holder 102.

FIG. 3 is a perspective view of a combustion system 300, according to an embodiment. The burner system 300 includes a pilot burner 104 and main fuel nozzles 106. Though not shown in FIG. 3, the pilot burner 104 is configured to support the pilot diffusion flame 108 by outputting the pilot fuel 112 through low velocity orifices. Though not shown in FIG. 3, the main fuel nozzles 106 are configured to support the main flame 110 by outputting the main fuel 114 as one or more high velocity streams or “jets”.

According to an embodiment, the pilot burner 104 is supported by and receives the pilot fuel 112 via a fuel pipe 220. The fuel pipe 220 extends into the furnace volume 101 via an opening 240 in a floor 238 of the furnace. A stiffener 222 can be positioned around the fuel pipe 220 to prevent the fuel pipe 220 from wobbling. The main fuel nozzles 106 also extend through the opening 240 in the floor 238. The main fuel nozzles 106 can be supported by fuel risers 224. In one embodiment, the main fuel nozzles 106 include orifices that output the main fuel with about a 2° spread.

According to an embodiment, the pilot burner 104 includes a fuel manifold having a plurality of segments 219 joined together, each segment 219 having a plurality of fuel orifices (e.g., fuel orifices 218 in FIGS. 2A, 2B) configured to pass fuel from inside the fuel manifold to the furnace volume 101. The plurality of segments 219 may be formed as respective tubes configured to freely pass the fuel delivered from the fuel pipe 220 into the fuel manifold. In one embodiment, at least a portion of the tubes is arranged as spokes radiating from a center disposed substantially at a centerline along the axis. In the embodiment of FIG. 3, the plurality of segments 219 are arranged in an “X” shape.

In one embodiment, each segment 219 includes one or more sections of reticulated ceramic 226 disposed in and supported by a “U” channel in the segments 219. The pilot fuel 112 can flow from the fuel orifices 218 into the channels or passageways of the one or more sections of reticulated ceramic 226. The pilot flame 108 can be held at least partially within the channels or passageways of the one or more sections of reticulated ceramic 226.

In one embodiment, the one or more sections of reticulated ceramic 226 are disposed superjacent to at least one tube of the tubes forming the plurality of segments 219. The at least one tube may define a plurality of fuel flow apertures disposed along a length of the at least one tube. In an embodiment, the at least one tube defines a plurality of fuel flow apertures configured to allow gaseous pilot fuel 112 to flow upward into a “U” channel formed superjacent to the at least one tube. In an embodiment, a distal flame holder 102 may be disposed at a third position along the fuel and combustion air flow axis, more distal from the main fuel nozzles 106 than the pilot burner 104. The distal flame holder 102 may include a perforated flame holder configured to control a flame length.

In one embodiment, the burner system 300 includes support legs 252. The support legs 252 can support a distal flame holder 102 (not shown in FIG. 3), including a solid bluff body and/or perforated flame holder, in the furnace volume 101 above the pilot burner 104. The distal flame holder can hold a portion of the main flame 110.

FIG. 4 is an illustration of a pilot burner 104 in the shape of an H, according to an embodiment. The pilot burner 104 includes a plurality of fuel orifices 218 that can output the pilot fuel 112. The pilot burner 104 can be made up of a plurality of tubes segment joined together to form the “H” shape.

FIG. 5 is an illustration of a pilot burner 104 in the shape of a spiral, according to an embodiment. The pilot burner 104 includes a plurality of fuel orifices 218 that can output the pilot fuel 112. The pilot burner 104 can be made up of a plurality of tubes segment joined together to form the spiral shape.

FIG. 6 is an illustration of a pilot burner 104 in the shape of a hexagon with sides attached to a center hub, according to an embodiment. The pilot burner 104 includes a plurality of fuel orifices 218 that can output the pilot fuel 112. The pilot burner 104 can be made up of a plurality of tubes segment joined together to form the shape shown in FIG. 6.

FIG. 7 is a simplified diagram of a combustion system 700 including a distal flame holder 102 configured to hold a combustion reaction wherein the distal flame holder 102 includes a perforated flame holder, according to an embodiment. The distal flame holder 102 can be implemented in the burner systems 111, 200, and 300, according to various embodiments. As used herein, the terms distal flame holder, bluff body flame holder, perforated flame holder, perforated reaction holder, porous flame holder, porous reaction holder, duplex, and duplex tile shall be considered synonymous or interchangeable unless context dictates otherwise, or further definition is provided. FIGS. 7-10B specifically describe a burner system employing a perforated flame holder as one type of distal flame holder 102.

Experiments performed by the inventors have shown that distal flame holders 102 described herein can support very clean combustion. Specifically, in experimental use of burner systems 700 ranging from pilot scale to full scale, output of oxides of nitrogen (NOx) was measured to range from low single digit parts per million (ppm) down to undetectable (less than 1 ppm) concentration of NOx at the stack. These remarkable results were measured at 3% (dry) oxygen (O₂) concentration with undetectable carbon monoxide (CO) at stack temperatures typical of industrial furnace applications (1400-1600° F.). Moreover, these results did not require any extraordinary measures such as selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), water/steam injection, external flue gas recirculation (FGR), or other heroic extremes that may be required for conventional burners to even approach such clean combustion.

According to embodiments, the burner system 700 includes a fuel and oxidant source 702 disposed to output main fuel and oxidant into a combustion volume 704 to form a main fuel and oxidant mixture 706. The fuel and oxidant source can include the main fuel nozzles 106. As used herein, the terms fuel and oxidant mixture and fuel stream may be used interchangeably and considered synonymous depending on the context, unless further definition is provided. As used herein, the terms combustion volume, combustion chamber, furnace volume, and the like shall be considered synonymous unless further definition is provided. The perforated flame holder 102 is disposed in the combustion volume 704 and positioned to receive the main fuel and oxidant mixture 706.

FIG. 8 is a side sectional diagram 800 of a portion of a perforated flame holder as the distal flame holder 102 of FIG. 7, according to an embodiment. Referring to FIGS. 7 and 8, the perforated flame holder 102 includes a perforated flame holder body 708 defining a plurality of perforations 710 aligned to receive the main fuel and oxidant mixture 706 from the fuel and oxidant source 702. As used herein, the terms perforation, pore, aperture, elongated aperture, and the like, in the context of the perforated flame holder 102, shall be considered synonymous unless further definition is provided. The perforations 710 are configured to collectively hold a combustion reaction supported by the main fuel and oxidant mixture 706.

The fuel can include hydrogen, a hydrocarbon gas, a vaporized hydrocarbon liquid, an atomized hydrocarbon liquid, or a powdered or pulverized solid. The fuel can be a single species or can include a mixture of gas(es), vapor(s), atomized liquid(s), and/or pulverized solid(s). For example, in a process heater application the fuel can include fuel gas or byproducts from the process that include carbon monoxide (CO), hydrogen (H₂), and methane (CH₄). In another application the fuel can include natural gas (mostly CH₄) or propane (C₃H₈). In another application, the fuel can include #2 fuel oil or #6 fuel oil. Dual fuel applications and flexible fuel applications are similarly contemplated by the inventors. The oxidant can include oxygen carried by air, flue gas, and/or can include another oxidant, either pure or carried by a carrier gas. The terms oxidant and oxidizer shall be considered synonymous herein.

According to an embodiment, the perforated flame holder body 708 can be bounded by an input face 712 disposed to receive the main fuel and oxidant mixture 706, an output face 714 facing away from the fuel and oxidant source 702, and a peripheral surface 716 defining a lateral extent of the perforated flame holder 102. The plurality of perforations 710 which are defined by the perforated flame holder body 708 extend from the input face 712 to the output face 714. The plurality of perforations 710 can receive the main fuel and oxidant mixture 706 at the input face 712. The main fuel and oxidant mixture 706 can then combust in or near the plurality of perforations 710 and combustion products can exit the plurality of perforations 710 at or near the output face 714.

According to an embodiment, the perforated flame holder 102 is configured to hold a majority of the combustion reaction 802 within the perforations 710. For example, on a steady-state basis, more than half the molecules of fuel output into the combustion volume 704 by the fuel and oxidant source 702 may be converted to combustion products between the input face 712 and the output face 714 of the perforated flame holder 102. According to an alternative interpretation, more than half of the heat or thermal energy output by the combustion reaction 802 may be output between the input face 712 and the output face 714 of the perforated flame holder 102. As used herein, the terms heat, heat energy, and thermal energy shall be considered synonymous unless further definition is provided. As used above, heat energy and thermal energy refer generally to the released chemical energy initially held by reactants during the combustion reaction 802. As used elsewhere herein, heat, heat energy and thermal energy correspond to a detectable temperature rise undergone by real bodies characterized by heat capacities. Under nominal operating conditions, the perforations 710 can be configured to collectively hold at least 80% of the combustion reaction 802 between the input face 712 and the output face 714 of the perforated flame holder 102. In some experiments, the inventors produced a combustion reaction 802 that was apparently wholly contained in the perforations 710 between the input face 712 and the output face 714 of the perforated flame holder 102. According to an alternative interpretation, the perforated flame holder 102 can support combustion between the input face 712 and output face 714 when combustion is “time-averaged.” For example, during transients, such as before the perforated flame holder 102 is fully heated, or if too high a (cooling) load is placed on the system, the combustion may travel somewhat downstream from the output face 714 of the perforated flame holder 102. Alternatively, if the cooling load is relatively low and/or the furnace temperature reaches a high level, the combustion may travel somewhat upstream of the input face 712 of the perforated flame holder 102.

While a “flame” is described in a manner intended for ease of description, it should be understood that in some instances, no visible flame is present. Combustion occurs primarily within the perforations 710, but the “glow” of combustion heat is dominated by a visible glow of the perforated flame holder 102 itself. In other instances, the inventors have noted transient “huffing” or “flashback” wherein a visible flame momentarily ignites in a region lying between the input face 712 of the perforated flame holder 102 and the main fuel nozzle 718, within the dilution region DD. Such transient huffing or flashback is generally short in duration such that, on a time-averaged basis, a majority of combustion occurs within the perforations 710 of the perforated flame holder 102, between the input face 712 and the output face 714. In still other instances, the inventors have noted apparent combustion occurring downstream from the output face 714 of the perforated flame holder 102, but still a majority of combustion occurred within the perforated flame holder 102 as evidenced by continued visible glow from the perforated flame holder 102 that was observed.

The perforated flame holder 102 can be configured to receive heat from the combustion reaction 802 and output a portion of the received heat as thermal radiation 804 to heat-receiving structures (e.g., furnace walls and/or radiant section working fluid tubes) in or adjacent to the combustion volume 704. As used herein, terms such as radiation, thermal radiation, radiant heat, heat radiation, etc. are to be construed as being substantially synonymous, unless further definition is provided. Specifically, such terms refer to blackbody-type radiation of electromagnetic energy, primarily at infrared wavelengths, but also at visible wavelengths owing to elevated temperature of the perforated flame holder body 708.

Referring especially to FIG. 8, the perforated flame holder 102 outputs another portion of the received heat to the main fuel and oxidant mixture 706 received at the input face 712 of the perforated flame holder 102. The perforated flame holder body 708 may receive heat from the combustion reaction 802 at least in heat receiving regions 806 of perforation walls 808. Experimental evidence has suggested to the inventors that the position of the heat receiving regions 806, or at least the position corresponding to a maximum rate of receipt of heat, can vary along the length of the perforation walls 808. In some experiments, the location of maximum receipt of heat was apparently between ⅓ and ½ of the distance from the input face 712 to the output face 714 (i.e., somewhat nearer to the input face 712 than to the output face 714). The inventors contemplate that the heat receiving regions 806 may lie nearer to the output face 714 of the perforated flame holder 102 under other conditions. Most probably, there is no clearly defined edge of the heat receiving regions 806 (or for that matter, the heat output regions 810, described below). For ease of understanding, the heat receiving regions 806 and the heat output regions 810 will be described as particular regions 806, 810.

The perforated flame holder body 708 can be characterized by a heat capacity. The perforated flame holder body 708 may hold thermal energy from the combustion reaction 802 in an amount corresponding to the heat capacity multiplied by temperature rise and transfer the thermal energy from the heat receiving regions 806 to heat output regions 810 of the perforation walls 808. Generally, the heat output regions 810 are nearer to the input face 712 than are the heat receiving regions 806. According to one interpretation, the perforated flame holder body 708 can transfer heat from the heat receiving regions 806 to the heat output regions 810 via thermal radiation, depicted graphically as 804. According to another interpretation, the perforated flame holder body 708 can transfer heat from the heat receiving regions 806 to the heat output regions 810 via heat conduction along heat conduction paths 812. The inventors contemplate that multiple heat transfer mechanisms including conduction, radiation, and possibly convection may be operative in transferring heat from the heat receiving regions 806 to the heat output regions 810. In this way, the perforated flame holder 102 may act as a heat source to maintain the combustion reaction 802, even under conditions where a combustion reaction 802 would not be stable when supported from a conventional flame holder.

The inventors believe that the perforated flame holder 102 causes the combustion reaction 802 to begin within thermal boundary layers 814 formed adjacent to walls 808 of the perforations 710. Insofar as combustion is generally understood to include a large number of individual reactions, and since a large portion of combustion energy is released within the perforated flame holder 102, it is apparent that at least a majority of the individual reactions occur within the perforated flame holder 102. As the relatively cool main fuel and oxidant mixture 706 approaches the input face 712, the flow is split into portions that respectively travel through individual perforations 710. The hot perforated flame holder body 708 transfers heat to the fluid, notably within thermal boundary layers 814 that progressively thicken as more and more heat is transferred to the incoming main fuel and oxidant mixture 706. After reaching a combustion temperature (e.g., the auto-ignition temperature of the fuel), the reactants continue to flow while a chemical ignition delay time elapses, over which time the combustion reaction 802 occurs. Accordingly, the combustion reaction 802 is shown as occurring within the thermal boundary layers 814. As flow progresses, the thermal boundary layers 814 merge at a merger point 816. Ideally, the merger point 816 lies between the input face 712 and output face 714 that define the ends of the perforations 710. At some position along the length of a perforation 710, the combustion reaction 802 outputs more heat to the perforated flame holder body 708 than it receives from the perforated flame holder body 708. The heat is received at the heat receiving region 806, is held by the perforated flame holder body 708, and is transported to the heat output region 810 nearer to the input face 712, where the heat is transferred into the cool reactants (and any included diluent) to bring the reactants to the ignition temperature.

In an embodiment, each of the perforations 710 is characterized by a length L defined as a reaction fluid propagation path length between the input face 712 and the output face 714 of the perforated flame holder 102. As used herein, the term reaction fluid refers to matter that travels through a perforation 710. Near the input face 712, the reaction fluid includes the main fuel and oxidant mixture 706 (optionally including nitrogen, flue gas, and/or other “non-reactive” species). Within the combustion reaction 802 region, the reaction fluid may include plasma associated with the combustion reaction 802, molecules of reactants and their constituent parts, any non-reactive species, reaction intermediates (including transition states), and reaction products. Near the output face 714, the reaction fluid may include reaction products and byproducts, non-reactive gas, and excess oxidant.

The plurality of perforations 710 can be each characterized by a transverse dimension D between opposing perforation walls 808. The inventors have found that stable combustion can be maintained in the perforated flame holder 102 if the length L of each perforation 710 is at least four times the transverse dimension D of the perforation. In other embodiments, the length L can be greater than six times the transverse dimension D. For example, experiments have been run where L is at least eight, at least twelve, at least sixteen, and at least twenty-four times the transverse dimension D. Preferably, the length L is sufficiently long for thermal boundary layers 814 to form adjacent to the perforation walls 808 in a reaction fluid flowing through the perforations 710 to converge at merger points 816 within the perforations 710 between the input face 712 and the output face 714 of the perforated flame holder 102. In experiments, the inventors have found L/D ratios between 12 and 48 to work well (i.e., produce low NOx, produce low CO, and maintain stable combustion).

The perforated flame holder body 708 can be configured to convey heat between adjacent perforations 710. The heat conveyed between adjacent perforations 710 can be selected to cause heat output from the combustion reaction portion 802 in a first perforation 710 to supply heat to stabilize a combustion reaction portion 802 in an adjacent perforation 710.

Referring especially to FIG. 7, the fuel and oxidant source 702 can further include a main fuel nozzle 718 (e.g., corresponding to main fuel nozzle(s) 106 described herein), configured to output main fuel 114, and an oxidant source 720 configured to output a fluid including the oxidant. For example, the main fuel nozzle 718 can be configured to output substantially pure fuel (as opposed to, e.g., a fuel-air mixture). The oxidant source 720 can be configured to output combustion air carrying oxygen, and optionally, flue gas.

The perforated flame holder 102 can be held by a perforated flame holder support structure 722 configured to hold the perforated flame holder 102 at a dilution distance DD away from the main fuel nozzle 718. The main fuel nozzle 718 can be configured to emit a fuel jet selected to entrain the oxidant to form the main fuel and oxidant mixture 706 as the fuel jet and oxidant travel along a path to the perforated flame holder 102 through the dilution distance DD between the main fuel nozzle 718 and the perforated flame holder 102. Additionally, or alternatively (particularly when a blower is used to deliver oxidant contained in combustion air), the oxidant or combustion air source 720 can be configured to entrain the fuel and the fuel and oxidant travel through the dilution distance DD. In some embodiments, a flue gas recirculation path 724 can be provided. Additionally, or alternatively, the main fuel nozzle 718 can be configured to emit a fuel jet selected to entrain the oxidant and to entrain flue gas as the fuel jet travels through the dilution distance DD between the main fuel nozzle 718 and the input face 712 of the perforated flame holder 102.

The main fuel nozzle 718 can be configured to emit the fuel through one or more fuel orifices 726 having an inside diameter dimension that is referred to as “nozzle diameter.” The perforated flame holder support structure 722 can support the perforated flame holder 102 to receive the main fuel and oxidant mixture 706 at the distance DD away from the main fuel nozzle 718 greater than 20 times the nozzle diameter. In another embodiment, the perforated flame holder 102 is disposed to receive the main fuel and oxidant mixture 706 at the distance DD away from the main fuel nozzle 718 between 100 times and 1100 times the nozzle diameter. Preferably, the perforated flame holder support structure 722 is configured to hold the perforated flame holder 102 at a distance about 200 times or more of the nozzle diameter away from the main fuel nozzle 718. When the main fuel and oxidant mixture 706 travels about 200 times the nozzle diameter or more, the mixture is sufficiently homogenized to cause the combustion reaction 802 to produce minimal NOx.

The fuel and oxidant source 702 can alternatively include a premix fuel and oxidant source, according to an embodiment. A premix fuel and oxidant source can include a premix chamber (not shown), a fuel nozzle configured to output fuel into the premix chamber, and an oxidant (e.g., combustion air) channel configured to output the oxidant into the premix chamber. A flame arrestor can be disposed between the premix fuel and oxidant source and the distal flame holder 102 and be configured to prevent flame flashback into the premix fuel and oxidant source. Alternatively, as described herein, a pilot burner, pilot flame holder, and/or continuous pilot flame may be disposed between the fuel and oxidant source 702 and the distal flame holder 102 to ensure combustion of the main fuel and the oxidant.

The oxidant source 720, whether configured for entrainment in the combustion volume 704 or for premixing, can include a blower configured to force the oxidant through the fuel and oxidant source 702.

The perforated flame holder support structure 722 can be configured to support the perforated flame holder 102 from a floor or wall (not shown) of the combustion volume 704, for example. In another embodiment, the perforated flame holder support structure 722 supports the perforated flame holder 102 from the fuel and oxidant source 702. Alternatively, the perforated flame holder support structure 722 can suspend the perforated flame holder 102 from an overhead structure (such as a flue, in the case of an up-fired system). The perforated flame holder support structure 722 can support the perforated flame holder 102 in various orientations and directions.

The perforated flame holder 102 can include a single perforated flame holder body 708. In another embodiment, the perforated flame holder 102 can include a plurality of adjacent perforated flame holder sections that collectively provide a tiled perforated flame holder 102.

The perforated flame holder support structure 722 can be configured to support the plurality of perforated flame holder sections. The perforated flame holder support structure 722 can include a metal superalloy, a cementitious, and/or ceramic refractory material. In an embodiment, the plurality of adjacent perforated flame holder sections can be joined with a fiber reinforced refractory cement.

The perforated flame holder 102 can have a width dimension W between opposite sides of the peripheral surface 716 at least twice a thickness dimension T between the input face 712 and the output face 714. In another embodiment, the perforated flame holder 102 can have a width dimension W between opposite sides of the peripheral surface 716 at least three times, at least six times, or at least nine times the thickness dimension T between the input face 712 and the output face 714 of the perforated flame holder 102.

In an embodiment, the perforated flame holder 102 can have a width dimension W less than a width of the combustion volume 704. This can allow the flue gas recirculation path 724 from above to below the perforated flame holder 102 to lie between the peripheral surface 716 of the perforated flame holder 102 and the combustion volume wall (not shown).

Referring again to both FIGS. 7 and 8, the perforations 710 can be of various shapes. In an embodiment, the perforations 710 can include elongated squares, each having a transverse dimension D between opposing sides of the squares. In another embodiment, the perforations 710 can include elongated hexagons, each having a transverse dimension D between opposing sides of the hexagons. In yet another embodiment, the perforations 710 can include hollow cylinders, each having a transverse dimension D corresponding to a diameter of the cylinder. In another embodiment, the perforations 710 can include truncated cones or truncated pyramids (e.g., frustums), each having a transverse dimension D radially symmetric relative to a length axis that extends from the input face 712 to the output face 714. In some embodiments, the perforations 710 can each have a lateral dimension D equal to or greater than a quenching distance of the flame based on standard reference conditions. Alternatively, the perforations 710 may have lateral dimension D less than a standard reference quenching distance.

In one range of embodiments, each of the plurality of perforations 710 has a lateral dimension D between 0.05 inch and 1.0 inch. Preferably, each of the plurality of perforations 710 has a lateral dimension D between 0.1 inch and 0.5 inch. For example, the plurality of perforations 710 can each have a lateral dimension D of about 0.2 to 0.4 inch.

The void fraction of a perforated flame holder 102 is defined as the total volume of all perforations 710 in a section of the perforated flame holder 102 divided by a total volume of the perforated flame holder 102 including perforated flame holder body 708 and perforations 710. The perforated flame holder 102 should have a void fraction between 0.10 and 0.90. In an embodiment, the perforated flame holder 102 can have a void fraction between 0.30 and 0.80. In another embodiment, the perforated flame holder 102 can have a void fraction of about 0.70. Using a void fraction of about 0.70 was found to be especially effective for producing very low NOx.

The perforated flame holder 102 can be formed from a fiber reinforced cast refractory material and/or a refractory material such as an aluminum silicate material. For example, the perforated flame holder 102 can be formed to include mullite or cordierite. Additionally, or alternatively, the perforated flame holder body 708 can include a metal superalloy such as Inconel or Hastelloy. The perforated flame holder body 708 can define a honeycomb. Honeycomb is an industrial term of art that need not strictly refer to a hexagonal cross section and most usually includes cells of square cross section. Honeycombs of other cross-sectional areas are also known.

The inventors have found that the perforated flame holder 102 can be formed from VERSAGRID® ceramic honeycomb, available from Applied Ceramics, Inc. of Doraville, S.C.

The perforations 710 can be parallel to one another and normal to the input and output faces 712, 714. In another embodiment, the perforations 710 can be parallel to one another and formed at an angle relative to the input and output faces 712, 714. In another embodiment, the perforations 710 can be non-parallel to one another. In another embodiment, the perforations 710 can be non-parallel to one another and non-intersecting. In another embodiment, the perforations 710 can be intersecting. The perforated flame holder body 708 can be one piece or can be formed from a plurality of sections.

In another embodiment, which is not necessarily preferred, the perforated flame holder 102 may be formed from reticulated ceramic material. The term “reticulated” refers to a netlike structure. Reticulated ceramic material is often made by dissolving a slurry into a sponge of specified porosity, allowing the slurry to harden, and burning away the sponge and curing the ceramic.

In another embodiment, which is not necessarily preferred, the perforated flame holder 102 may be formed from a ceramic material that has been punched, bored or cast to create channels.

In another embodiment, the perforated flame holder 102 can include a plurality of tubes or pipes bundled together. The plurality of perforations 710 can include hollow cylinders and can optionally also include interstitial spaces between the bundled tubes. In an embodiment, the plurality of tubes can include ceramic tubes. Refractory cement can be included between the tubes and configured to adhere the tubes together. In another embodiment, the plurality of tubes can include metal (e.g., superalloy) tubes. The plurality of tubes can be held together by a metal tension member circumferential to the plurality of tubes and arranged to hold the plurality of tubes together. The metal tension member can include stainless steel, a superalloy metal wire, and/or a superalloy metal band.

The perforated flame holder body 708 can alternatively include stacked perforated sheets of material, each sheet having openings that connect with openings of subjacent and superjacent sheets. The perforated sheets can include perforated metal sheets, ceramic sheets and/or expanded sheets. In another embodiment, the perforated flame holder body 708 can include discontinuous packing bodies such that the perforations 710 are formed in the interstitial spaces between the discontinuous packing bodies. In one example, the discontinuous packing bodies include structured packing shapes. In another example, the discontinuous packing bodies include random packing shapes. For example, the discontinuous packing bodies can include ceramic Raschig ring, ceramic Berl saddles, ceramic Intalox saddles, and/or metal rings or other shapes (e.g., Super Raschig Rings) that may be held together by a metal cage.

The inventors contemplate various explanations for why burner systems including the perforated flame holder 102 provide such clean combustion.

According to an embodiment, the perforated flame holder 102 may act as a heat source to maintain a combustion reaction 802 even under conditions where a combustion reaction 802 would not be stable when supported by a conventional flame holder. This capability can be leveraged to support combustion using a leaner fuel-to-oxidant mixture than is typically feasible. Thus, according to an embodiment, at the point where the fuel stream 706 contacts the input face 712 of the perforated flame holder 102, an average fuel-to-oxidant ratio of the fuel stream 706 is below a (conventional) lower combustion limit of the fuel component of the fuel stream 706—lower combustion limit defines the lowest concentration of fuel at which a main fuel and oxidant mixture 706 will burn when exposed to a momentary ignition source under normal atmospheric pressure and an ambient temperature of 25° C. (77° F.).

The perforated flame holder 102 and systems including the perforated flame holder 102 and/or other distally placed flame holder described herein were found to provide substantially complete combustion of CO (single digit ppm down to undetectable, depending on experimental conditions), while supporting low NOx. According to one interpretation, such a performance can be achieved due to a sufficient mixing used to lower peak flame temperatures (among other strategies). Flame temperatures tend to peak under slightly rich conditions, which can be evident in any diffusion flame that is insufficiently mixed. By sufficiently mixing, a homogenous and slightly lean mixture can be achieved prior to combustion. This combination can result in reduced flame temperatures, and thus reduced NOx formation. In one embodiment, “slightly lean” may refer to 3% O₂, i.e., an equivalence ratio of ˜0.87. Use of even leaner mixtures is possible but may result in elevated levels of O₂. Moreover, the inventors believe perforation walls 808 may act as a heat sink for the combustion fluid. This effect may alternatively or additionally reduce combustion temperatures and lower NOx.

According to another interpretation, production of NOx can be reduced if the combustion reaction 802 occurs over a very short duration of time. Rapid combustion causes the reactants (including oxygen and entrained nitrogen) to be exposed to NOx-formation temperature for a time too short for NOx formation kinetics to cause significant production of NOx. The time required for the reactants to pass through the perforated flame holder 102 is very short compared to a conventional flame. The low NOx production associated with perforated flame holder combustion may thus be related to the short duration of time required for the reactants (and entrained nitrogen) to pass through the perforated flame holder 102.

FIG. 9 is a flow chart showing a method 900 for operating a burner system including the distal flame holder 102 (e.g., a perforated flame holder) shown and described herein, according to an embodiment. To operate a burner system including a distal flame holder, the distal flame holder is first heated to a temperature sufficient to maintain combustion of the fuel and oxidant mixture.

According to a simplified description, the method 900 begins with step 902, wherein the distal flame holder is preheated to a start-up temperature, T_(S). After the distal flame holder is raised to the start-up temperature, the method proceeds to step 904, wherein the fuel and oxidant are provided to the distal flame holder and combustion is held by the distal flame holder.

According to a more detailed description, step 902 begins with step 906, wherein start-up energy is provided at the distal flame holder. Simultaneously or following providing start-up energy, a decision step 908 determines whether the temperature T of the distal flame holder is at or above the start-up temperature, T_(S). As long as the temperature of the distal flame holder is below its start-up temperature, the method loops between steps 906 and 908 within the preheat step 902. In decision step 908, if the temperature T of at least a predetermined portion of the distal flame holder is greater than or equal to the start-up temperature, the method 900 proceeds to overall step 904, wherein fuel and oxidant is supplied to and combustion is held by the distal flame holder.

Step 904 may be broken down into several discrete steps, at least some of which may occur simultaneously.

Proceeding from decision step 908, a fuel and oxidant mixture is provided to the distal flame holder, as shown in step 910. The fuel and oxidant may be provided by a fuel and oxidant source that includes a separate fuel nozzle and oxidant (e.g., combustion air) source, for example. In this approach, the fuel and oxidant are output in one or more directions selected to cause the fuel and oxidant mixture to be received by the input face of the distal flame holder. The fuel may entrain the combustion air (or alternatively, the combustion air may dilute the fuel) to provide a fuel and oxidant mixture at the input face of the distal flame holder at a fuel dilution selected for a stable combustion reaction that can be held within the perforations of the distal flame holder.

Proceeding to step 912, the combustion reaction is held by the distal flame holder.

In step 914, heat may be output from the distal flame holder. The heat output from the distal flame holder may be used to power an industrial process, heat a working fluid, generate electricity, or provide motive power, for example.

In optional step 916, the presence of combustion may be sensed. Various sensing approaches have been used and are contemplated by the inventors. Generally, combustion held by the distal flame holder is very stable and no unusual sensing requirement is placed on the system. Combustion sensing may be performed using an infrared sensor, a video sensor, an ultraviolet sensor, a charged species sensor, thermocouple, thermopile, flame rod, and/or other combustion sensing apparatuses. In an additional or alternative variant of step 916, a pilot flame or other ignition source may be provided to cause ignition of the fuel and oxidant mixture in the event combustion is lost at the distal flame holder.

Proceeding to decision step 918, if combustion is sensed not to be stable, the method 900 may exit to step 924, wherein an error procedure is executed. For example, the error procedure may include turning off fuel flow, re-executing the preheating step 902, outputting an alarm signal, igniting a stand-by combustion system, or other steps. If, in decision step 918, combustion in the distal flame holder is determined to be stable, the method 900 proceeds to decision step 920, wherein it is determined if combustion parameters should be changed. If no combustion parameters are to be changed, the method loops (within step 904) back to step 910, and the combustion process continues. If a change in combustion parameters is indicated, the method 900 proceeds to step 922, wherein the combustion parameter change is executed. After changing the combustion parameter(s), the method loops (within step 904) back to step 910, and combustion continues.

Combustion parameters may be scheduled to be changed, for example, if a change in heat demand is encountered. For example, if less heat is required (e.g., due to decreased electricity demand, decreased motive power requirement, or lower industrial process throughput), the fuel and oxidant flow rate may be decreased in step 922. Conversely, if heat demand is increased, then fuel and oxidant flow may be increased. Additionally, or alternatively, if the combustion system is in a start-up mode, then fuel and oxidant flow may be gradually increased to the distal flame holder over one or more iterations of the loop within step 904.

Referring again to FIG. 7, the burner system 700 includes a heater 728 operatively coupled to the distal flame holder 102. As described in conjunction with FIGS. 8 and 9, the distal flame holder 102 operates by outputting heat to the incoming main fuel and oxidant mixture 706. After combustion is established, this heat is provided by the combustion reaction 802; but before combustion is established, the heat is provided by the heater 728.

Various heating apparatuses have been used and are contemplated by the inventors. In some embodiments, the heater 728 can include a flame holder configured to support a flame disposed to heat the distal flame holder 102. The fuel and oxidant source 702 can include a main fuel nozzle 718 configured to emit a fuel stream 706 and an oxidant source 720 configured to output oxidant (e.g., combustion air) adjacent to the fuel stream 706. The main fuel nozzle 718 and oxidant source 720 can be configured to output the fuel stream 706 to be progressively diluted by the oxidant (e.g., combustion air). The distal flame holder 102 can be disposed to receive a diluted main fuel and oxidant mixture 706 that supports a combustion reaction 802 that is stabilized by the distal flame holder 102 when the distal flame holder 102 is at an operating temperature. A start-up flame holder, in contrast, can be configured to support a start-up flame at a location corresponding to a relatively unmixed fuel and oxidant mixture that is stable without stabilization provided by the heated distal flame holder 102.

The burner system 700 can further include a controller 730 operatively coupled to the heater 728 and to a data interface 732. For example, the controller 730 can be configured to control a start-up flame holder actuator configured to cause the start-up flame holder to hold the start-up flame when the distal flame holder 102 needs to be pre-heated and to not hold the start-up flame when the distal flame holder 102 is at an operating temperature (e.g., when T≥T_(S)).

Various approaches for actuating a start-up flame are contemplated. In one embodiment, the start-up flame holder includes a mechanically actuated distal configured to be actuated to intercept the main fuel and oxidant mixture 706 to cause heat-recycling and/or stabilizing vortices and thereby hold a start-up flame; or to be actuated to not intercept the main fuel and oxidant mixture 706 to cause the main fuel and oxidant mixture 706 to proceed to the distal flame holder 102. In another embodiment, a fuel control valve, blower, and/or damper may be used to select a main fuel and oxidant mixture 706 flow rate that is sufficiently low for a start-up flame to be jet-stabilized; and upon reaching a distal flame holder 102 operating temperature, the flow rate may be increased to “blow out” the start-up flame. In another embodiment, the heater 728 may include an electrical power supply operatively coupled to the controller 730 and configured to apply an electrical charge or voltage to the main fuel and oxidant mixture 706. An electrically conductive start-up flame holder may be selectively coupled to a voltage ground or other voltage selected to attract the electrical charge in the main fuel and oxidant mixture 706. The attraction of the electrical charge was found by the inventors to cause a start-up flame to be held by the electrically conductive start-up flame holder.

In another embodiment, the heater 728 may include an electrical resistance heater configured to output heat to the distal flame holder 102 and/or to the main fuel and oxidant mixture 706. The electrical resistance heater 728 can be configured to heat up the distal flame holder 102 to an operating temperature. The heater 728 can further include a power supply and a switch operable, under control of the controller 730, to selectively couple the power supply to the electrical resistance heater 728.

An electrical resistance heater 728 can be formed in various ways. For example, the electrical resistance heater 728 can be formed from KANTHAL® wire (available from Sandvik Materials Technology division of Sandvik AB of Hallstahammar, Sweden) threaded through at least a portion of the perforations 710 defined by the distal flame holder body 708. Alternatively, the heater 728 can include an inductive heater, a high-energy beam heater (e.g., microwave or laser), a frictional heater, electro-resistive ceramic coatings, or other types of heating technologies.

Other forms of start-up apparatuses are contemplated. For example, the heater 728 can include an electrical discharge igniter or hot surface igniter configured to output a pulsed ignition to the oxidant and fuel. Additionally, or alternatively, a start-up apparatus can, as discussed in greater detail herein, include a pilot flame apparatus disposed to ignite the main fuel and oxidant mixture 706 that would otherwise enter the distal flame holder 102. The electrical discharge igniter, hot surface igniter, and/or pilot flame apparatus can be operatively coupled to the controller 730, which can cause the electrical discharge igniter or pilot flame apparatus to maintain combustion of the main fuel and oxidant mixture 706 in or upstream from the distal flame holder 102 before the distal flame holder 102 is heated sufficiently to maintain combustion.

The burner system 700 can further include a sensor 734 operatively coupled to the controller 730. The sensor 734 can include a heat sensor configured to detect infrared radiation or a temperature of the distal flame holder 102. The control circuit 730 can be configured to control the heater 728 responsive to input from the sensor 734. Optionally, a fuel control valve 736 can be operatively coupled to the controller 730 and configured to control a flow of fuel to the fuel and oxidant source 702. Additionally, or alternatively, an oxidant blower or damper 738 can be operatively coupled to the controller 730 and configured to control flow of the oxidant (or combustion air).

The sensor 734 can further include a combustion sensor operatively coupled to the control circuit 730, the combustion sensor being configured to detect a temperature, video image, and/or spectral characteristic of a combustion reaction 802 held by the distal flame holder 102. The fuel control valve 736 can be configured to control a flow of fuel from a fuel source to the fuel and oxidant source 702. The controller 730 can be configured to control the fuel control valve 736 responsive to input from the combustion sensor 734. The controller 730 can be configured to control the fuel control valve 736 and/or oxidant blower or damper 738 to control a preheat flame type of heater 728 to heat the distal flame holder 102 to an operating temperature. The controller 730 can similarly control the fuel control valve 736 and/or the oxidant blower or damper 738 to change the main fuel and oxidant mixture 706 flow responsive to a heat demand change received as data via the data interface 732.

FIG. 10A is a simplified perspective view of a combustion system 1000, including another alternative distal flame holder 102, according to an embodiment. The distal flame holder 102 is a reticulated ceramic perforated flame holder configured to hold a combustion reaction, according to an embodiment. FIG. 10B is a simplified side sectional diagram of a portion of the reticulated ceramic perforated flame holder 102 of FIG. 10A, according to an embodiment. The distal flame holder 102 of FIGS. 10A, 10B can be implemented in the various combustion systems described herein, according to an embodiment. The distal flame holder 102 is configured to support a combustion reaction (e.g., combustion reaction 802 of FIG. 8) of the main fuel and oxidant mixture 706 received from the fuel and oxidant source 702 at least partially within the distal flame holder 102. According to an embodiment, the distal flame holder 102 can be configured to support a combustion reaction of the main fuel and oxidant mixture 706 upstream, downstream, within, and adjacent to the reticulated ceramic perforated flame holder 102.

According to an embodiment, the perforated flame holder body 708 can include reticulated fibers 1039. The reticulated fibers 1039 can define branching perforations 710 that weave around and through the reticulated fibers 1039. According to an embodiment, the perforations 710 are formed as passages between the reticulated fibers 1039.

According to an embodiment, the reticulated fibers 1039 are formed as a reticulated ceramic foam. According to an embodiment, the reticulated fibers 1039 are formed using a reticulated polymer foam as a template. According to an embodiment, the reticulated fibers 1039 can include alumina silicate. According to an embodiment, the reticulated fibers 1039 can be formed from extruded mullite or cordierite. According to an embodiment, the reticulated fibers 1039 can include Zirconia. According to an embodiment, the reticulated fibers 1039 can include silicon carbide.

As mentioned above, the term “reticulated fibers” refers to a netlike structure. According to an embodiment, the reticulated fibers 1039 are formed from an extruded ceramic material. In reticulated fiber 1039 embodiments, the interaction between the main fuel and oxidant mixture 706, the combustion reaction 802, and heat transfer to and from the perforated flame holder body 708 can function similarly to the embodiment shown and described above with respect to FIGS. 7-9. One difference in activity is a mixing between perforations 710, because the reticulated fibers 1039 form a discontinuous perforated flame holder body 708 that allows flow back and forth between neighboring perforations 710.

According to an embodiment, the network of reticulated fibers 1039 is sufficiently open for downstream reticulated fibers 1039 to emit radiation for receipt by upstream reticulated fibers 1039 for the purpose of heating the upstream reticulated fibers 1039 sufficiently to maintain combustion of a main fuel and oxidant mixture 706. Compared to a continuous perforated flame holder body 708, heat conduction paths (such as heat conduction paths 812 in FIG. 8) between reticulated fibers 1039 are reduced due to separation of the reticulated fibers 1039. This may cause relatively more heat to be transferred from a heat-receiving region or area (such as heat receiving region 806 in FIG. 8) to a heat-output region or area (such as heat-output region 810 of FIG. 8) of the reticulated fibers 1039 via thermal radiation (shown as element 804 in FIG. 8).

According to an embodiment, individual perforations 710 may extend between an input face 712 to an output face 714 of the perforated flame holder 102. Perforations 710 may have varying lengths L. According to an embodiment, because the perforations 710 branch into and out of each other, individual perforations 710 are not clearly defined by a length L.

According to an embodiment, the perforated flame holder 102 is configured to support or hold a combustion reaction (see element 802 of FIG. 8) or a flame at least partially between the input face 712 and the output face 714. According to an embodiment, the input face 712 corresponds to a surface of the perforated flame holder 102 proximal to the main fuel nozzle 718 or to a surface that first receives fuel. According to an embodiment, the input face 712 corresponds to an extent of the reticulated fibers 1039 proximal to the main fuel nozzle 718. According to an embodiment, the output face 714 corresponds to a surface distal to the main fuel nozzle 718 or opposite the input face 712. According to an embodiment, the input face 712 corresponds to an extent of the reticulated fibers 1039 distal to the main fuel nozzle 718 or opposite to the input face 712.

According to an embodiment, the formation of thermal boundary layers 814, transfer of heat between the perforated flame holder body 708 and the gases flowing through the perforations 710, a characteristic perforation width dimension D, and the length L can each be regarded as related to an average or overall path through the perforated reaction holder 102. In other words, the dimension D can be determined as a root-mean-square of individual Dn values determined at each point along a flow path. Similarly, the length L can be a length that includes length contributed by tortuosity of the flow path, which may be somewhat longer than a straight-line distance TRH from the input face 712 to the output face 714 through the perforated reaction holder 102. According to an embodiment, the void fraction (expressed as (total perforated reaction holder 102 volume−reticulated fiber 1039 volume)/total volume)) is about 70%.

According to an embodiment, the reticulated ceramic perforated flame holder 102 is a tile about 1″×4″×4″. According to an embodiment, the reticulated ceramic perforated flame holder 102 includes about 100 pores per square inch of surface area. Other materials and dimensions can also be used for a reticulated ceramic perforated flame holder 102 in accordance with principles of the present disclosure.

According to an embodiment, the reticulated ceramic perforated flame holder 102 can include shapes and dimensions other than those described herein. For example, the perforated flame holder 102 can include reticulated ceramic tiles that are larger or smaller than the dimensions set forth above. Additionally, the reticulated ceramic perforated flame holder 102 can include shapes other than generally cuboid shapes.

According to an embodiment, the reticulated ceramic perforated flame holder 102 can include multiple reticulated ceramic tiles. The multiple reticulated ceramic tiles can be joined together such that each ceramic tile is in direct contact with one or more adjacent reticulated ceramic tiles. The multiple reticulated ceramic tiles can collectively form a single perforated flame holder 102. Alternatively, each reticulated ceramic tile can be considered a distinct perforated flame holder 102.

FIG. 11 is a flow chart showing a method 1100 for operating a distal flame holder burner system, according to an embodiment. According to an embodiment, the method 1100 includes, in operation 1102, providing heat to a distal flame holder (102) from a pilot flame supported by a pilot burner, the pilot flame being fueled by a pilot fuel, the distal flame holder and the pilot burner being disposed in a furnace and in proximity to one another. The pilot burner may be disposed between the distal flame holder and one or more main fuel nozzles, and a distance between the pilot burner and the distal flame holder is smaller than a distance between the pilot burner and the one or more main fuel nozzles. Operation 1104 includes introducing mixed main fuel and air to the distal flame holder. Operation 1106 includes holding at least a portion of a combustion reaction of the mixed main fuel and air within the distal flame holder while the pilot burner continues to support the pilot flame.

Operation 1102 may include providing the pilot fuel to the pilot burner from a pilot fuel source, controlling a pilot fuel rate of flow, and emitting the pilot fuel from a plurality of orifices (e.g., 218 in FIGS. 2A, 2B, 4-6) of the pilot burner. The orifices may be disposed across a breadth of the mixed main fuel and air. The operation may further include igniting the mixed main fuel and air at the pilot burner with the pilot flame.

According to an embodiment, the method 1100 may further include measuring a temperature of the distal flame holder, and, when the temperature of the distal flame holder is at or above a predetermined threshold, reducing a pilot fuel rate of flow to reduce a size of the pilot flame. Reducing the size of the pilot flame relative to the size of the combustion reaction of the mixed main fuel and air may cause a reduction of emissions of oxides of nitrogen.

In operation 1104, the introducing the mixed main fuel and air to the distal flame holder may include introducing at a proximal end of a mixing tube, the main fuel via the one or more main fuel nozzles, and the air. The proximal end of the mixing tube may be disposed proximate to the one or more main fuel nozzles, while the distal end of the mixing tube may be disposed proximate to the distal flame holder. The mixing tube may be open from its proximal end to its distal end.

According to an embodiment, the method 1100 may further include educing a flue gas into the proximal end of the mixing tube.

In an embodiment, the pilot burner may be disposed between the distal flame holder and the distal end of the mixing tube.

FIG. 12 is a flow chart showing a method 1200 for operating a burner system, according to an embodiment. According to an embodiment, the method 1200 includes, in operation 1202, supporting a diffusion flame across a portion of a width of a furnace volume at a position distal from a furnace floor. Operation 1204 includes providing combustion air to the furnace volume from a location near the furnace floor. Operation 1206 includes outputting a high-pressure main fuel jet from each of one or more main fuel nozzles at one or more locations near the furnace floor. Operation 1208 includes mixing the main fuel with the combustion air while the main fuel and combustion air travel from the locations near the furnace floor to the distal position. Operation 1210 includes combusting the main fuel by exposing the mixed main fuel and air to the diffusion flame.

According to an embodiment, the method 1200 may further include holding a main flame, resulting from combusting the main fuel, at a stable position with a distal flame holder disposed more distal from the furnace floor than the diffusion flame.

In an embodiment, in operation 1202, the supporting of a diffusion flame may include producing and holding the diffusion flame at a pilot burner, the pilot burner being disposed in the furnace volume between the one or more main fuel nozzles and the distal flame holder. The pilot burner may be disposed in closer proximity to the distal flame holder than it is to the furnace floor. In another embodiment, the holding of the diffusion flame at the pilot burner may include supplying a pilot fuel to the pilot burner, emitting the pilot fuel from one or more pilot fuel orifices of the pilot burner, and maintaining ignition of a mixture of combustion air and the emitted pilot fuel at the pilot burner to support the diffusion flame. The one or more pilot fuel orifices may constitute a plurality of pilot fuel orifices disposed across the portion of the width of the furnace volume sufficiently wide to cause contact of the diffusion flame with the mixture of the main fuel and combustion air across a width of mixed main fuel and combustion air. Additionally, and/or alternatively, the one or more pilot fuel orifices may include a plurality of pilot fuel orifices having a collective area sufficiently large to support the diffusion flame at a low momentum. The holding the diffusion flame at the pilot burner may include controlling a rate of supply of a pilot fuel to the pilot burner.

According to an embodiment, the method 1200 may further include detecting a temperature of a distal flame holder disposed above the diffusion flame, and when the distal flame holder reaches at least a predetermined threshold temperature, reducing a rate of supply of a pilot fuel to a pilot burner supporting the diffusion flame.

According to an embodiment, the method 1200 may further include detecting the combusting of the main fuel at the distal flame holder using a an electrocapacitive sensor, the electrocapacitive sensor configured to output sensor signals to a controller.

According to an embodiment, a combustion system may include an oxidant source configured to output an oxidant into a furnace volume. A pilot burner may be configured to support a pilot flame by outputting a pilot fuel to support a pilot diffusion flame at least during a preheating state, and a main fuel nozzle may be configured to output a main fuel into the furnace volume from a proximal position during a standard operating state at least after the preheating state is complete. The combustion system may include includes a distal flame holder positioned in the furnace volume to be preheated by the pilot flame during the preheating state and to hold a combustion reaction of the main fuel and oxidant adjacent to the distal flame holder during the standard operating state, and a combustion sensor configured to sense a condition of the distal flame holder and to generate a sensor signal indicative of the condition of the distal flame holder. The combustion system further includes one or more actuators configured to adjust a flow of the main fuel from the main fuel nozzle, to adjust a flow of the pilot fuel to the pilot burner, and to adjust a flow of the oxidant from the oxidant source. A controller is communicatively coupled to the actuators and the combustion sensor. The controller may be configured to receive the sensor signals from the combustion sensor and to control the actuators to adjust the flow of the pilot fuel, the main fuel, and the oxidant responsive to the sensor signals and in accordance with software instructions stored in a non-transitory computer readable medium coupled to the controller.

According to an embodiment, the combustion system further includes a pilot flame sensor configured to sense a condition of a pilot flame and to output a sensor signal indicative of the condition of the pilot flame. The combustion sensor may include the pilot flame sensor. In one embodiment, the pilot flame sensor may include an electrocapacitive sensor, an electro-resistive sensor, and/or a tomographic sensor (e.g., employing electrocapacitive tomography (ECT).)

According to an embodiment, the combustion system further includes an ignitor configured to generate an arc. The controller may be configured to control one or more of the actuators to cause the ignitor to generate the arc to ignite the pilot flame if the electrocapacitive sensor indicates that the pilot flame is not present and all safety interlocks are met. In an embodiment, the controller is configured to adjust a size of the pilot flame response to the sensor signals from at least the combustion sensor by controlling one or more of the actuators to adjust the flow of the pilot fuel or the oxidant. The combustion sensor may be configured to detect the combustion reaction at the distal flame holder and to output sensor signals to the controller responsive to a detected state of the combustion reaction. In an embodiment, the combustion sensor is configured for operation as a flashback sensor configured to detect a flashback of the combustion reaction toward the main fuel nozzle from the distal flame holder. The combustion sensor may include an electrocapacitive sensor.

FIG. 13A is a diagram of a combustion system 1300 including a distal flame holder 102 and an electrocapacitive (EC) sensor 1305, according to an embodiment. FIG. 13B is a top view of the distal flame holder 102 and an electrocapacitive sensor 1305, according to an embodiment.

In an embodiment, the EC sensor may be configured as an electrocapacitive tomography (ECT) sensor. Alternatively, a combustion sensor may be configured as an electro-resistive or electro-conductive sensor by modifying the signal processing. As used herein, the term electrocapacitive will be understood to refer also to electro-resistive or electro-conductive sensors. An EC sensor is essentially a simplified ECT sensor in that it has fewer electrodes and can sense combustion presence, but not necessarily a specific location of combustion. It will be understood that references to ECT sensors similarly refer to EC sensors.

As used herein, the term electrocapacitive tomography (ECT) shall be understood as described. ECT sensing may be fundamentally capacitive or may additionally or alternatively be made to measure a conductance, a resistance, an impedance, or other electrical parameter. ECT may include a plurality of sensor channels, such as may be produced by moving a sensor through different positions or by using a sensor array, such as may be seen in more common (e.g., medical) tomography systems. Additionally, or alternatively, an ECT system may include a range of sensor channels as few as a single channel defined by two electrodes positioned relative to a sensed region (e.g., a flame-holding region, a blow-off region, a flash-back region, a flue gas region, a pilot flame region, etc.). Unless context dictates otherwise, disclosure and claims herein shall be accorded this broad meaning.

In an embodiment, the combustion system 1300 may include a combustion sensor that includes an EC sensor or ECT device, such as the electrocapacitive sensor 1305 of FIG. 13A and FIG. 13B. The electrocapacitive sensor 1305 can include a first set of electrodes 1320, including multiple pairs of electrodes 1320, positioned laterally around the distal flame holder 102 in order to sense a parameter of the distal flame holder 102. The electrocapacitive sensor 1305 can also include a second set of electrodes positioned upstream from the distal flame holder 102. The first set of electrodes 1320 can sense a capacitance or other parameter in a vicinity of the distal flame holder 102. The second set of electrodes can sense a capacitance or other parameter upstream from the distal flame holder 102, for example at a pilot burner (e.g., 104 in FIG. 3) or upstream from the pilot burner. The controller 730 can compare the capacitance or other parameter sensed by the first set of electrodes 1320 to the capacitance or other parameter sensed by the second set of electrodes in order to detect a combustion reaction parameter, presence of flashback, etc.

In an embodiment, the combustion system 1300 may include a distal flame holder sensor that includes the electrocapacitive sensor 1305. The distal flame holder sensor can share use of the first set of electrodes 1320 described in relation to the combustion sensor. In this case, the first set of electrodes 1320 including pairs of electrodes positioned laterally around the distal flame holder 102 can act as both the electrocapacitive sensor 1305, and at least a portion of the combustion sensor.

In one embodiment, the combustion system 1300 may include a pilot flame sensor (not shown) that includes the electrocapacitive sensor 1305. The pilot flame sensor and the combustion sensor can share use of electrodes positioned upstream from the distal flame holder 102 or laterally around the distal flame holder 102. Two or more of the pilot flame sensors, the combustion sensor, and the distal flame holder sensor can share electrodes 1320 of an electrocapacitive sensor 1305.

The combustion system 1300 includes a fuel and oxidant source 702, a distal flame holder 102, a controller 730, an electrocapacitive tomography device 1305, and a memory 1307. The fuel and oxidant source 702 can include main fuel nozzle(s) (e.g., the main fuel nozzles 106 described above) and the oxidant source 720. Additionally, the fuel and oxidant source 702 can include a pilot burner, such as the pilot burner 104 described above.

According to an embodiment, the fuel and oxidant source 702 includes, for example, a fuel nozzle configured to output the main fuel and oxidant onto the distal flame holder 102. The distal flame holder 102 holds a combustion reaction of the fuel and oxidant primarily adjacent to and/or within the distal flame holder 102.

According to an embodiment, the electrocapacitive sensor 1305 may be configured as an image capture device (e.g., using ECT) that includes a plurality of electrodes 1320 positioned at selected locations adjacent to the distal flame holder 102. The electrocapacitive sensor 1305 is configured to make images of the distal flame holder 102 based on the capacitance between the electrodes 1320. The images represent slices of the distal flame holder 102 based on the capacitances between the electrodes 1320. The capacitance between pairs of electrodes 1320 depends, in part, on the dielectric constant of the material(s) between the pairs of electrodes 1320. In particular, the dielectric constant within the perforations of the distal flame holder 102 can change based on the characteristics of the combustion reaction within the perforations. Therefore, the images produced by the electrocapacitive sensor 1305 can give an indication of a temperature within the perforations or a concentration or flow of fuel, oxidant, and flue gasses at various locations corresponding to the distal flame holder 102 based on the dielectric constant at the various locations of the distal flame holder 102. The controller 730 can analyze the images and adjust the combustion reaction based on the images.

According to an embodiment, the controller 730 is configured to cause the electrocapacitive sensor 1305 to capture one or more images of the combustion reaction. In one embodiment, the controller 730 is further configured to analyze the one or more images and to adjust the characteristics of the combustion reaction based on the analysis of the one or more images.

FIG. 13B is a top view of the distal flame holder 102 and the electrocapacitive sensor 1305, according to an embodiment. The electrocapacitive sensor 1305 includes multiple pairs of electrodes 1320 positioned laterally around the distal flame holder 102. Each pair of electrodes 1320 includes two electrodes 1320 opposite one another, with the distal flame holder 102 positioned between the pair of electrodes 1320 or in a fringing field therebetween. The controller 730 controls each pair of electrodes 1320 to make a plurality of images (or an aggregate image) of the distal flame holder 102, according to an embodiment.

In one embodiment, the electrodes 1320 a and 1320 a are a pair, the electrodes 1320 b and 1320 b are a pair, the electrodes 1320 c and 1320 c are a pair, the electrodes 1320 d and 1320 d are a pair, the electrodes 1320 e and 1320 e are a pair, and the electrodes 1320 f and 1320 f are a pair. The electrocapacitive sensor 1305 can generate electrocapacitive tomography images based on a capacitance between the pairs of electrodes 1320.

In one embodiment, the plurality of electrodes 1320 includes one or more first pairs of electrodes 1320 separated from each other by the distal flame holder 102 and disposed opposite each other in a first orientation substantially perpendicular to a primary direction of a flow of the main fuel toward the distal flame holder 102. In one example, the first pairs of electrodes 1320 can include the pair of electrodes 1320 a and the pair of electrodes 1320 b. The first pairs of electrodes 1320 a and 1320 b sense the capacitance of the distal flame holder 102 along an X direction substantially perpendicular to a primary direction of flow of the main fuel and the oxidant toward the distal flame holder 102. The primary direction of flow of the main fuel and the oxidant toward the distal flame holder 102 can correspond to a Z direction.

In one embodiment, the plurality of electrodes 1320 includes one or more second pairs of electrodes separated from each other by the distal flame holder 102 and disposed opposite each other in a second orientation substantially perpendicular to both the first orientation and the primary direction of the flow of the main fuel. In one example, the second pairs of electrodes 1320 can include the pair of electrodes 1320 c and the pair of electrodes 1320 d. The second pairs of electrodes 1320 c and 1320 d sense the capacitance of the distal flame holder 102 along a Y direction substantially perpendicular to the primary direction of flow of the main fuel and the oxidant and substantially perpendicular to orientation of the first pairs of electrodes 1320 a and 1320 b.

In one embodiment, the plurality of electrodes 1320 can include pairs of electrodes 1320 oriented transverse to both the first pairs of electrodes 1320 a and 1320 b and the second pairs of electrodes 1320 c and 1320 d. The transverse pairs of electrodes 1320 can include the pair of electrodes 1320 e and the pair of electrodes 1320 f.

Although the views of FIG. 13A and FIG. 13B have shown the electrocapacitive sensor 1305 including the electrodes 1320 positioned laterally around the distal flame holder 102, an electrocapacitive sensor in accordance with principles of the present disclosure can include pairs of electrodes positioned in configurations other than laterally around a distal flame holder 102. An electrocapacitive sensor can include pairs of electrodes positioned upstream from the distal flame holder 102, downstream from the distal flame holder 102, or in other locations depending on the particular aspect of a combustion system that the electrocapacitive sensor is intended to sense or monitor. Accordingly, other sensors described or contemplated in relation to embodiments disclosed herein (e.g., sensors 734, 1414) can include electrocapacitive sensors where suitable.

FIG. 14 is a diagram of a low emissions modular burner system 1400 including one or more burner modules 1401 according to an embodiment. Each burner module 1401 may include a main fuel source, separately valved from all other fuel sources, configured to selectively deliver a main fuel stream 1404 for dilution by a flow of combustion air. The main fuel source may, in certain embodiments, correspond to or be implemented to include one or more main fuel nozzles 1402. Each burner module 1401 may include a main fuel igniter 1406 configured to cause ignition of the main fuel stream 1404 emitted from the main fuel nozzle(s) 1402. Each burner module 1401 may respectively include a distal flame holder 102 configured to hold a combustion reaction supported by the main fuel stream 1404 when the distal flame holder 102 is at or above a predetermined temperature. In an embodiment, the predetermined temperature may be equal to or greater than a main fuel auto-ignition temperature. Each burner module 1401 may include a pilot burner 1408 configured to pre-heat the distal flame holder 102 to the predetermined temperature, according to an embodiment. In an embodiment, the pilot burner 1408 of each burner module 1401 may include a continuous pilot burner that also may operate to ignite the main fuel. In some embodiments, the distal flame holder 102 may be separated from the main fuel nozzle(s) 1402 and from the pilot burner 1408 by respective non-zero distances (D1, D2).

The low emissions modular burner system 1400 may include a common combustion air source 1405 configured to provide combustion air to each of the one or more burner modules 1401, and a wall 1407 encircling all of the one or more burner modules 1401, the wall 1407 being configured to laterally contain combustion fluids corresponding to the one or more burner modules 1401. In an embodiment, the distal flame holder 102 is configured to hold a combustion reaction supported by the main fuel stream 1404 when the distal flame holder 102 is at or above the main fuel auto-ignition temperature. In one embodiment, each pilot burner 1408 includes a continuous pilot burner. In an embodiment, the pilot burner 1408 is configured selectively to output heat at any of a plurality of heating rates. At least one heating rate may be selected to cause a rise in sensible temperature of the distal flame holder 102 to the predetermined operating temperature, and at least one other heating rate may be selected to cause the pilot burner 1408 to maintain a pilot flame function while a majority of total fuel consumed per unit of time is provided by the main fuel source 1402. In an embodiment, the common combustion air source 1405 is configured to provide natural draft combustion air to each burner module (1401) of the one or more burner modules 1401.

FIG. 15 is a diagram of a control circuit 1512 for use in the control system 1412 of FIG. 14, according to an embodiment.

According to an embodiment, referring to FIGS. 14 and 15, the low emissions modular burner system 1400 further includes one or more separate main fuel valves 1410 for each burner module 1401, each main fuel valve 1410 including separate main fuel valve actuators configured to operate responsive to receiving control signals, and further includes a control system 1412 configured to output respective control signals to each of the separate main fuel valve actuators. In an embodiment, the control system 1412 further includes an interface 1514 (see FIG. 15) between the control system 1412 and an input channel. The input channel may include a physical (e.g., electrically conductive) connection or a wireless connection. Accordingly, the interface 1514 may include a network interface and/or a hardware interface such as, but not limited to, a USB interface, a PID controller interface, a relay interface, a radio interface, a WiFi interface, a Bluetooth interface, etc. In an embodiment, the interface 1514 includes an interface to one or more sensors disposed to sense physical parameters related to each burner module 1401 and environs. Sensors (e.g., 1414 in FIG. 14) and operation thereof, may include capacitance coupled (e.g., patch) electrodes (which may alternatively be referred to as antennas) cooperating to emit and receive a radio frequency signal across a region intended to hold a combustion reaction. A change in capacitance corresponds to a change in charged species concentration, which has been found to be covariant with the presence or absence of combustion. According to an embodiment, the electrodes may be disposed in sufficient number, and be positioned, to provide a tomographic scan of the combustion region. The sensors 1414 may be connected to the control system 1412 by a communication channel 1415. In some embodiments the communication channel may wired (e.g., electrically conductive). In instances where the sensors 1414 employ electrocapacitive measurements and/or electrocapacitive tomography, the communication channel 1415 may provide a voltage and/or current to electrodes of the sensors 1414.

The interface 1514 may be configured to receive a signal corresponding to a burner capacity requirement. The control system 1412 may further include one or more burner module sensor inputs 1515 a, 1515 b, each of the one or more burner module sensor inputs 1515 a, 1515 b being configured to receive a signal corresponding to a burner module status, wherein the burner module status is provided by sensor hardware 1414. The control system 1412 may further include a microcontroller or other logic processor 1516, a computer readable memory 1518, and a module sequencer 1520 (which may optionally be embodied as or by the logic processor 1516 and the computer readable memory 1518 when executing module sequencing functions) configured to select a subset of the one or more burner modules 1401 for ignition. The control system 1412 may further include a respective one or more main fuel valve driver outputs, each operatively coupled to one of the separate main fuel valve actuators 1410. In an embodiment, the one or more burner module sensor inputs 1515 a, 1515 b are configured to receive input from one or more sensors, such as the sensor hardware 1414, or from one or more sensors external to the burner module(s) (1401). The one or more sensors may include a demand sensor including one or more of a condensate pressure sensor, a heating energy demand sensor, and a condensate presence sensor.

In an embodiment having more than one burner module 1401, a control circuit 1512 (see FIG. 15) may include a module sequencer 1520. The module sequencer 1520 may include a state machine configured to changeably sequence an actuation of the one or more burner modules 1401. For example, it may be desirable to periodically change an assignment of the burner modules to different positions in an actuation sequence in accord with demand. In one embodiment, a last module turned on in the previous module sequence may also operate as the first/only module turned on during a turn-down state. In another embodiment, the assignment of burner modules 1401 need not be identical with respect to capacity, age (e.g., cycle count), and design. A start-up sequence may be at least partially identical with each base demand/surge capacity cycle. The inventors contemplate various arrangements, actuation sequences, and selections of the assignment of burner modules 1401 may offer specific advantages to particular application characteristics.

According to an embodiment, the low emissions modular burner system 1400 further includes a run sequencer 1522. In an embodiment, the control circuit 1512 may include the run sequencer 1522. For a given module in an actuation queue served by the module sequencer 1520, the run sequencer 1522 may include a state machine configured to sequence steps in a burner module start-up schedule for one or more of the burner modules 1401. Start-up schedules may be stored in the memory 1518 and periodically updated via the interface 1514 that includes a network interface. Illustrative methods and aspects for start-up sequencing are described with respect to several of the other figures included herein.

According to an embodiment, the control circuit 1512 (of control system 1412 of FIG. 14) of the low emissions modular burner system 1400 further includes an actuator driver module 1524. In an embodiment, the control circuit 1512 may include the actuator driver module 1524. The actuator driver module 1524 may be configured to provide the respective control signals to each of the separate valve actuators. The actuator driver module 1524 may include a state machine configured to load a driver shift register enable bit for amplification by a power module 1526, responsive to data from a start sequencer. Signals to/from the power module 1526 may be respectively coupled to actuatable main fuel valve(s) 1410, via connection(s) 1411. Similarly, actuatable pilot fuel valve(s) 1416 may be respectively coupled to the power module 1526 via connection(s) 1417.

Sensors 1414 are described herein as performing sensory functions or functioning as signal outputs. In embodiments that require outputting a signal at sensors/electrodes 1414, the power module 1526 may be employed to amplify such signal, e.g., for the aforementioned emission and receipt of radio frequency signals across a combustion region. In an alternative embodiment, sensors 1414 may provide a signal for generating data, e.g., a flame tomogram. In such embodiment dedicated sensor inputs 1515 a, 1515 b may be utilized. In yet another alternative embodiment, the sensors 1414 may provide a subset of many data signals that communicate via interface 1514 of the control circuitry 1512. As described above, the interface 1514 may provide wireless or wired connections using various communication protocols, which may permit the sensors 1414 to communicate via a standard method such as USB, WiFi, ethernet, or the like.

According to an embodiment, the low emissions modular burner system 1400 may further include a demand module 1528. In an embodiment, demand for system capacity is received in substantially real time via a network interface included in the interface 1514. The demand module 1528 may be configured to supervise automatic operation of the one or more burner module(s) 1401 selectively based on at least one of a stored schedule and a received demand signal.

The demand module 1528 may consist essentially of a data value in a register of the memory 1518. In another embodiment, especially in systems where real time data access via the interface 1514 is not guaranteed, the demand module 1528 may include a real time clock and, as data, a scheduled system capacity. In an embodiment, the demand module 1528 may operate as a supervisory state configured to automatically operate the modular burner system 1400 according to seasonal and/or periodic demand dynamics. Similarly, in an environment with chaotic dynamic demand, operation of the interface 1514 may be more crucial. In systems characterized by chaotic fluctuations in capacity demand, the inventors contemplate that an interface with parallel or greater channel diversification and/or hardening may be advisable. Optionally, portions of the module sequencer 1520 may be virtualized and cloud accessed.

According to an embodiment, the logic processor 1516 is configured to read and execute computer executable instructions supported by a non-transitory computer readable memory 1518 to receive capacity input data corresponding to the burner capacity requirement signal, read module status sensor data from sensor(s) corresponding to at least one burner module to verify that a selected one or more of the burner module(s) 1401 is ready for firing, select the subset of the one or more burner modules 1401 for firing, and drive at least one of the separate main fuel valve (1410) actuators corresponding to the selected one or more burner module(s) to open so as to provide fuel to a combustion reaction supported by the one or more burner module(s) 1401.

According to an embodiment, the control system 1412 further includes a demand sensor. The demand sensor may include a heating energy demand sensor.

According to an embodiment, each burner module 1401 further includes a pilot fuel source configured to provide a pilot fuel, a pilot fuel igniter (e.g., 1406) configured to ignite a flow of the pilot fuel, and a distal pilot or start-up burner (e.g., constituting pilot burner 1408) configured to hold a pilot flame supported by the pilot fuel, a pilot fuel source flow rate being selected to provide a pilot flame sized to raise the temperature of the distal flame holder 102 to the pre-determined temperature. In an embodiment, the predetermined temperature is equal to or greater than a main fuel auto-ignition temperature. As used herein, the terms pilot, pilot burner, distal pilot and start-up burner and pre-heating apparatus shall be considered synonymous unless context dictates otherwise.

According to an embodiment, the main igniter 1406 comprises the distal pilot. According to another embodiment, the main igniter 1406 includes the distal flame holder 102 when the distal flame holder 102 is heated to the pre-determined temperature by the distal pilot 1408. That is, the fuel and combustion air may ignite on contact with the pre-heated distal flame holder 102 rather than by a separate igniter. According to an embodiment, the predetermined temperature is the main fuel auto-ignition temperature.

According to an embodiment, the distal pilot burner 1408 is configured to be controlled to provide the pilot flame sized to raise the distal flame holder 102 to the pre-determined temperature during a burner module start-up cycle, and to not provide the pilot flame sized to raise the distal flame holder 102 to the pre-determined temperature at times other than during the burner module start-up cycle. In one embodiment, the distal pilot burner 1408 is configured to decrease to a pilot flame capacity at times other than during the burner module start-up cycle. In another embodiment, the distal pilot burner 1408 is configured to stop supporting a combustion reaction at times other than during the burner module start-up cycle. Additionally, and/or alternatively, the distal pilot burner 1408 may be disposed adjacent to the distal flame holder 102, and the distal pilot burner 1408 is controlled to be decreased to a pilot flame capacity at times other than during the burner module start-up cycle. In another embodiment, the pilot burner 1408 is disposed adjacent to the distal flame holder 102, and the distal pilot output is selected to maintain a constant capacity at all times during operation. In an embodiment, the pilot burner 1408 is configured to guarantee combustion of the main fuel, e.g., when the distal flame holder 102 does not support a combustion reaction. The main fuel may be a hydrocarbon gas. The pilot fuel may be one or more of hydrogen, natural gas or propane. According to embodiments, the pilot fuel and the main fuel may consist essentially of the same fuel.

According to an embodiment, a modular burner 1401 includes a housing 1403 having a combustion air inlet at a base. Each burner module 1401 may include an inlet configured to be coupled to a main fuel supply and to receive combustion air via the housing 1403, a distal flame holder 102 positioned inside the housing 1403, and a main fuel nozzle 1402 configured to receive a flow of main fuel from the inlet, and to emit a main fuel stream toward the distal flame holder 102.

According to an embodiment, each of the one or more burner modules 1401 is configured to be freestanding, supported only by a coupling at the inlet.

In an embodiment, the burner module 1401 is configured to be coupled to the burner and to be supported thereby.

According to an embodiment, the main fuel nozzle is one of a plurality of main fuel nozzles 1402, each of the main fuel nozzles 1402 configured to receive a flow of the main fuel from the inlet, and to emit a main fuel stream 1404 toward a respective portion of the distal flame holder 102.

According to an embodiment, the modular burner 1401 further includes a plurality of the main fuel valves 1410 operatively coupled between a common fuel line and a respective one of the plurality of main fuel nozzles 1402 and configured to independently control operation of the respective main nozzle 1402. That is, whereas FIG. 14 illustrates a main fuel valve 1410 shared by each of the main fuel nozzles 1402, an embodiment may include a separate—and separately controllable—main fuel valve 1410 for each main fuel nozzle 1402.

According to an embodiment, the modular burner 1401 further includes a distal pilot burner 1408 positioned between the distal flame holder 102 and the main fuel nozzle(s) 1402 for each burner module. The modular burner 1401 may be a retrofit burner positioned within the housing 1407, the retrofit burner including the distal flame holder 102 and the main fuel nozzle(s) 1402. Each distal pilot burner 1408 may include a plurality of pilot nozzles arranged in an array. In another embodiment, the distal pilot burner 1408 is configured to support a pilot flame between the distal pilot burner 1408 and the distal flame holder 102.

The main fuel nozzle(s) 1402 may include an aperture having a size that is variable. The main fuel nozzle(s) 1402 may be configured to regulate a velocity of the main fuel stream. According to an embodiment, the modular burner 1401 further includes an actuator operatively coupled to the main fuel nozzle(s) 1402 and configured to control the size of the aperture. The main fuel nozzle(s) 1402 may each include a main nozzle outlet and a control element, the control element being positioned to occlude some portion of the main nozzle outlet, and wherein movement of the control element varies a degree to which the main nozzle outlet is occluded by the control element.

In an embodiment, each burner module has a heating capacity of about 8 MBtu/Hr.

FIG. 16 is a block diagram of a burner system 1600, according to an embodiment. The burner system 1600 includes a distal flame holder 1602 (corresponding to distal flame holder 102 described herein), a fuel and oxidant source 1620, and a mixing tube 1610. The fuel and oxidant source 1620 may include an oxidant conduit 1604 for delivery of an oxidant 1606 a, and one or more main fuel nozzle(s) 1618 for main delivery of a fuel 1606 b. The fuel 1606 b and the oxidant 1606 a mix in the mixing tube 1610 en route to the distal flame holder 1602, creating a fuel and oxidant mixture 1607. The distal flame holder 1602 is disposed and oriented to receive and (when at an operating temperature) to ignite the fuel and oxidant mixture 1607. The oxidant conduit 1604 provides a pathway for the oxidant 1606 a (e.g., air), and directs the oxidant 1606 a toward the distal flame holder 1602. The main fuel nozzle(s) 1618 direct the fuel 1606 b toward the distal flame holder 1602. The main fuel nozzle(s) 1618 may receive the fuel 1606 b from a fuel reservoir or pipeline (not shown, each or both referred to herein as a fuel supply) via a main fuel supply line 1608. The burner system 1600 may include a single main fuel nozzle 1618 or a plurality of the main fuel nozzle(s) 1618, each disposed and configured as described herein. The fuel 1606 b emitted by the main fuel nozzle(s) 1618, and the oxidant 1606 a emitted by the oxidant conduit 1604 become mixed as they travel toward the distal flame holder 1602. The fuel 1606 b and the oxidant 1606 a achieve a sufficiently uniform fuel and oxidant mixture 1607 (see also element 706 in FIG. 7) to permit efficient and uniform combustion within the distal flame holder 1602 at the operating temperature.

The burner system 1600 may include a pilot burner 1612. The pilot burner 1612 disposed proximate the distal flame holder and provides a pilot flame which may maintain ignition of the fuel and oxidant mixture 1607. In some implementations, the pilot burner 1612 may receive fuel from a pilot fuel supply line 1614. Alternatively, the pilot burner 1612 may be in fluid connection with the main fuel supply line 1608.

According to an embodiment, the mixing tube may be disposed a predetermined distance from a floor of the burner system and may be configured to receive at least the combustion air via the oxidant conduit 1604.

As described earlier in this disclosure, a source of flue gas diluent 1616 is contemplated. The inventors have observed that the introduction of a mixing tube facilitates a recirculation of flue gas—as a substantial flue gas diluent—from downstream of the distal flame holder 1602. The flue gas 1616 is educed to a proximal end (i.e., the main nozzle end) of the mixing tube 1610 by a flow of main fuel and combustion oxidant between the main fuel nozzle(s) 1618 and the distal flame holder 1602 through the mixing tube 1610. The recirculated flue gas 1616 mixes with the fuel and the combustion air before reaching the distal flame holder 1602. The non-reactive elements of the resulting mixture minimize a potential for flashback upstream from the distal flame holder 1602 while permitting additional combustion of the reactive elements of the flue gas, thus reducing, e.g., NOx and other potential pollutants.

Turning now to FIG. 17, a burner system 1700 may include a distal flame holder 1602, a plurality of main fuel nozzles 1618, one or more distal pilot burners 1704 (e.g., corresponding to the pilot nozzle(s) 1612), and a mixing tube 1710. The main fuel nozzles 1618 may be arranged in fluid connection with a main fuel source 1732. According to an embodiment, flow of main fuel from the main fuel source 1732 may be controlled via a main fuel control valve 1736. The one or more distal pilot burners 1704 may be arranged in fluid connection with a pilot fuel source 1730. According to an embodiment, flow of pilot fuel from the pilot fuel source 1730 may be controlled via a pilot fuel control valve 1734.

The distal pilot burner(s) 1704 may be configured to support a pilot flame by outputting a pilot fuel received via a pilot fuel pipe 1712 from the pilot fuel source 1730. The pilot fuel pipe 1712 may be disposed inside the mixing tube 1710 or—advantageously for maintenance, temperature regulation, etc.—outside the mixing tube 1710. In some embodiments, the pilot fuel pipe 1712 may form a portion of a support for the mixing tube 1710. According to an embodiment, the distal pilot burner(s) 1704 may be supported by and receive fuel via the pilot fuel pipe 1712. The pilot fuel pipe 1712 extends into the furnace volume 1701 via the opening 1740 in the floor 1738 of the furnace. Each distal pilot burner 1704 (corresponding to the pilot burner 104 described earlier) may include a pilot manifold formed in any of several shapes. For example, in FIG. 17, the pilot manifold is formed in a Y shape. See also the discussion above corresponding to, e.g., FIGS. 3-6 with respect to pilot burner configurations.

According to an embodiment, each distal pilot burner 1704 includes one or more manifolds that define a plurality of fuel orifices 1718 having a large collective area to collectively support a low momentum pilot flame (not shown). In an embodiment, the main fuel output by the main fuel nozzles 1618 and combustion air form a combustible mixture that expands in breadth as it flows from a proximal position of the main fuel nozzles 1618 to the distal position of the distal pilot burner(s) 1704. The plurality of fuel orifices 1718 may be disposed across the furnace volume 1701 sufficiently broadly to cause contact of the pilot flame with the main fuel and combustion air mixture across the breadth of the combustible mixture. In another embodiment, the main fuel nozzles 1618 may be configured to output fuel in co-flow with the air.

According to an embodiment, a distal pilot burner 1704 includes a fuel manifold having a plurality of segments 1719 joined together, each segment 1719 having a plurality of fuel orifices 1718 configured to pass fuel from inside the fuel manifold to the furnace volume 1701. The plurality of segments 1719 may be formed as respective tubes configured to freely pass the fuel delivered from the pilot fuel pipe 1712 into the fuel manifold. In one embodiment (e.g., as in FIG. 17), at least a portion of the tubes is arranged as spokes radiating from a center disposed substantially at a centerline along the axis. In another embodiment, at least a portion of the tubes is arranged as an “X”, a rectangle, an “H”, a wagon wheel, or a star.

According to an embodiment, a distal pilot burner 1704 includes a manifold including a curvilinear tube. In an embodiment, the curvilinear tube is arranged as a spiral, “

”, “

”, or “∞”.

According to an embodiment, the mixing tube 1712 may be arranged about a longitudinal axis of flow between the main fuel nozzles 1618 and the distal flame holder 1602. According to an embodiment, the mixing tube 1710 may include a bell-shaped or flared portion 1714 at an end proximate the main fuel nozzle 1618. The bell-shaped or flared portion 1714 may be disposed a predetermined distance from a floor 1738 of the burner system and may be configured to receive at least the combustion air via an opening 1740 in the floor 1738.

As described earlier in this disclosure, a source of flue gas diluent is contemplated. The inventors have observed that the introduction of a mixing tube facilitates a recirculation of flue gas—as a substantial flue gas diluent—from downstream of the distal flame holder 1602, and/or including combustion products of a pilot flame held at the pilot burner 1704. The flue gas is educed to a proximal end (i.e., the floor end) of the mixing tube 1710 by a flow of main fuel and combustion oxidant between the floor 1738 and the distal flame holder 1602 through the mixing tube 1710. The recirculated flue gas mixes with the fuel and the combustion air before reaching the distal flame holder 1602. The non-reactive elements of the resulting mixture minimize a potential for flashback upstream from the distal flame holder 1602 while permitting additional combustion of the reactive elements of the flue gas, thus reducing, e.g., NOx and other potential pollutants.

Those having skill in the art will recognize the FIG. 17 should not be relied upon as representing appropriate scale, relative dimensions, shapes, etc. For example, the mixing tube 1710 may have a diameter appropriate for providing a mixture of fuel and oxidant (e.g., fuel and oxidant mixture 1607) to at least most of the input face (e.g., input face 712 of FIG. 7) of the distal flame holder 1602. The opening at the proximal end of the mixing tube 1710, closest to the main fuel nozzles 1618, may have a largest diameter sized in correspondence to either the opening 1740 in the floor 1738 or sufficient to receive fuel input from each of the main fuel nozzles 1618. For example, in an embodiment that includes the bell-shaped or flared portion 1714, the largest diameter of the bell-shaped or flared portion 1714 may correspond to either the opening 1740 in the floor 1738 or may correspond to at least the farthest distance between main fuel nozzles 1618. A length of the mixing tube may be selected to permit sufficient time and/or distance for appropriate mixing of the fuel and the oxidant before reaching the distal flame holder 1602.

According to an embodiment, a burner system includes a distal flame holder configured to hold a combustion reaction of a fuel and an oxidant, and an oxidant conduit configured to direct the oxidant toward the distal flame holder. The burner system includes a main fuel nozzle oriented to direct a flow of a main fuel into a combustion volume for mixture with the oxidant in a dilution region between the main fuel nozzle and the distal flame holder when a temperature of the distal flame holder is above a predetermined temperature, and a mixing tube disposed in the dilution region, and being open from a mixing tube inlet to a mixing tube outlet between the main fuel nozzle and the distal flame holder, the mixing tube being formed to cause flow of the oxidant and fuel to educe flue gas into the mixing tube for mixing with fuel and oxidant.

In an embodiment, the mixing tube is configured to cause the flow of oxidant and fuel to form a flue gas recirculation path. The flue gas recirculation path may be external to the combustion chamber.

According to an embodiment, the burner system further includes a pilot burner configured to support a pilot flame between the outlet of the mixing tube and the distal flame holder.

In an embodiment, the mixing tube includes a flared portion at the mixing tube inlet. The flue gas recirculation path may include at least a toroidal volume between the mixing tube and a wall of the combustion volume. The flue gas may be educed into the fuel and oxidant stream at the mixing tube inlet for dilution of the fuel and oxidant stream.

According to an embodiment, the burner system further a continuous pilot disposed adjacent to the distal flame holder, the continuous pilot being configured to heat the distal flame holder to the predetermined temperature, and a controller operatively coupled to the main fuel source, the controller configured to receive an indication of a temperature of the distal flame holder and to control the flow of the main fuel responsive to the indication of the temperature.

According to an embodiment, the burner system further includes a mixing tube support structure configured to support the mixing tube, the mixing tube support structure configured to be supported by a surface defining the combustion volume.

FIG. 18 is an illustration showing a horizontally fired burner system 1800 including a distal pilot burner 1804 and a mixing tube 1810, according to an embodiment. The inventors have observed, in a variety of furnace applications, undesirable combustion oscillations occurring between a distal flame holder 1802 and a fuel and oxidant (combustion air) source 1820. Although not necessarily restricted to a confined furnace configuration—e.g., a water heater, boiler, or once-through steam generator (OTSG)—such applications are representative environments that can permit such combustion oscillations.

When fuel and oxidant are in sufficiently combustible proportion and exposed to sufficient heat for ignition, they can undesirably ignite upstream of the distal flame holder 1802. This phenomenon tends to oscillate and is referred to herein as “flashback,” and is sometimes colloquially referred to as “huffing.” In some implementations, insufficiently and/or non-uniformly cooled oxidant, e.g., flue gas, can be recirculated from, e.g., downstream of the distal flame holder 1802, resulting in a fuel-oxidant mixture with a sufficiently high temperature that the mixture may ignite prior to reaching the distal flame holder 1802. The flashback reduces the efficiency of the burner 1800 at least in part because heat from this premature combustion is not (in a gas-fired burner) radiant heat, is not sufficiently absorbed by the distal flame holder 1802 and/or boiler tubes and is thus wasted. Combustion products from the flashback can dilute the mixture and thus temporarily snuff the flashback combustion. Hence the oscillating nature of flashback.

The distal pilot burner 1804 may be configured for preheating of the distal flame holder 1802 and/or to address undesirable flashback by providing a constant and/or controllable ignition source for the fuel and combustion air mixture at a position sufficiently near to the distal flame holder 1802 to provide heat benefits from a diffusion pilot flame 1808 to the distal flame holder 1802.

The distal pilot burner 1804 may be disposed adjacent to the distal flame holder 1802 in a combustion volume 1801. The distal flame holder 1802 may be formed of a plurality of columns including refractory materials. In an embodiment, the distal pilot burner 1804 is configured to maintain a diffusion pilot flame 1808 during combustion of main fuel in a combustion reaction held by the distal flame holder 1802. The main fuel and the combustion air may be supplied by the fuel and combustion air source 1820 disposed a distance upstream from the distal pilot burner 1804. Accordingly, in an embodiment, the distance between main fuel nozzles 1806 of the fuel and combustion air source 1820 and the distal pilot burner 1804 may be at least 50 times a diameter of the main fuel nozzles 1806, at least 100 times a diameter of the main fuel nozzles 1806, or at least 200 times the diameter of the main fuel nozzles 1806.

According to an embodiment, the horizontally fired burner system 1800 may include a mixing tube 1810 disposed between the fuel and combustion air source 1820 and the distal pilot burner 1804. The mixing tube 1810 may include a flared portion at an opening proximal to the fuel and combustion air source 1820. The mixing tube 1810 directs a flow of fuel and combustion air from the fuel and combustion air source 1820 toward the distal pilot burner 1804 and the distal flame holder 1802. Flue gas 1816 may be recirculated outside the mixing tube 1810 to enter the proximal end thereof for mixture with the fuel and the combustion air.

According to another embodiment, the horizontally fired burner system 1800 includes the distal pilot burner 1804 disposed adjacent to the plurality of columns. The distal pilot burner 1804 may be configured to successively provide a pre-heating flame to raise a temperature of the distal flame holder 1802 to at least an auto-ignition temperature of main fuel prior to introduction of the main fuel, and to maintain the diffusion pilot flame 1808 during combustion of the main fuel in the combustion reaction held by the distal flame holder 1802. The distal pilot burner 1804 may be configured to support a large combustion reaction during pre-heating of the distal flame holder 1802 and to support a smaller combustion reaction during subsequent combustion of the main fuel.

A horizontally fired burner system disclosed may include a controller 1812 (corresponding to, e.g., control system 1412 described herein) configured to receive sensor inputs, e.g., from sensor 1815, and to control output of fuel and combustion air. In embodiments corresponding to FIG. 18, the controller 1812 may control use of the distal pilot burner 1804. For example, the controller 1812 may control an actuator (not shown) that controls rate and/or amount of fuel provided to the distal pilot burner 1804 based on, e.g., sensor inputs showing temperature of the distal flame holder 1802, presence or absence or quality of a flame at the distal flame holder 1802 and/or at the distal pilot burner 1804.

Those of skill in the art will recognize, in light of the present disclosure, that the combustion system in accordance with principles of the present disclosure can include sensors and actuators other than those disclosed herein, other combinations of sensors and actuators, as well as other kinds of actions to be taken by the controller (e.g., 730 of FIG. 7) responsive to sensor signals. All such other sensors, actuators, combinations, and actions fall within the scope of the present disclosure.

FIG. 19 illustrates a burner system 1900 according to an embodiment. The burner system 1900 includes a furnace 1904 (indicated schematically by a dashed-line envelope), in which a pilot burner 1902 is disposed at a distal position, where “distal” and “proximal” are defined as relative locations along a direction parallel to a main fuel and combustion air flow axis 1906. The “proximal” position near the bottom of FIG. 19. One or more main fuel nozzles 1908 may be disposed at a proximal position. The pilot burner 1902 may be a pre-mix burner configured to support a pilot flame PF using a pre-mixture of pilot fuel and an oxidant, while the one or more main fuel nozzles 1908 are configured to support a main flame MF in contact with the pilot flame PF. The pilot burner 1902 may be disposed such as to cause the main fuel and combustion air to be ignited by the pilot flame PF. Owing to the flow and mixing of main fuel and combustion air as the main fuel and combustion air pass from the proximal position to the distal position, and the position of flame holders 1930, the main flame MF is generally located more distal from the main fuel nozzles 1908 than the pilot flame PF, such that the pilot flame PF and distal flame holders 1930 define a flame front (i.e., farthest downward extent) of the main flame MF.

The pilot fuel and the main fuel may be a same fuel, or of a same fuel type, or may be distinct and of different types, according to embodiments.

In an embodiment, the pilot burner 1902 may include a pilot pre-mix chamber 1910 and a pilot fuel line with a fuel fitting 1921 configured to output the pilot fuel into the pilot pre-mix chamber 1910, as well as an oxidant channel configured to output the oxidant into the pilot pre-mix chamber. The pilot pre-mix chamber 1910 may be configured to mix the pilot fuel and the oxidant to produce the pre-mixed pilot fuel.

The pilot burner 1902 includes a premixed fuel nozzle 1912 configured to direct the pre-mixture and pilot flame PF toward an intended distal position. The distal position may be coincident with a flame holder 1930. The pilot burner 1902 may optionally include a flame arrestor disposed between the pilot pre-mix chamber and the pre-mix fuel nozzle 1912. In an embodiment, the flame arrestor may be disposed at an output of the pilot pre-mix chamber. In another embodiment, the flame arrestor may be disposed between the output of a premixed fuel tube and the pilot fuel nozzle. In another embodiment, the flame arrestor is omitted and flashback is controlled by maintaining a high mixture velocity from the pilot premix chamber 1910 and the pilot nozzle 1912. The pilot burner 1902 may include a housing cowl disposed above the output of the pilot nozzle.

In an embodiment, the burner system 1900 may include a fuel pipe 1920 configured to support the pilot burner 1902 at the distal position. In an embodiment, the fuel pipe 1920 may direct the pilot fuel to the fuel fitting 1921 such that the pilot fuel is released and mixed with oxidant within the pre-mix pipe 1910 as it approaches the pilot fuel nozzle 1912, i.e., the pre-mix pipe 1910 may act as a pre-mix chamber along a length in which pre-mix fuel and oxidant are both available. Alternatively, the pre-mix pipe 1910 may include a portion that acts as a pre-mix chamber near its distal end, as shown in FIG. 19. In another embodiment, the fuel pipe 1920 may direct pre-mixed pilot fuel and oxidant from a pre-mix chamber located outside the furnace 1904 or closer to the floor 1928 of the furnace to a pilot burner distal assembly. An outer support 1924 may be configured to substantially prevent wobbling of the fuel pipe 1920.

A pilot igniter may be configured to ignite the pre-mixture of the pilot fuel and the oxidant. The pilot igniter may be disposed downstream from the pilot nozzle to ignite the pre-mixture of pilot fuel and oxidant after the pilot fuel and oxidant exit the pilot fuel nozzle. The pilot igniter 1926 may include a spark generator configured to generate a spark to ignite the pre-mixture, or may include a hot surface igniter configured to heat up responsive to application of electrical energy to a temperature equal to or greater than an autoignition temperature of the pre-mixture.

The main flame MF, incorrectly shown, but inside a volume of the furnace 1904 in FIG. 19, may include a flame having a heat output of at least 10 times the heat output of the pilot flame PF when the burner system may be operating at a rated heat output. A rated heat output may correspond to operating in a steady state standard operating mode.

According to an embodiment, in the burner system 1900 the one or more main fuel nozzles 1908 may output a main fuel, where the main fuel supports the main flame. The one or more main fuel nozzles 1908 and combustion air may form a combustible mixture that expands in breadth as it flows from the proximal position to the distal position. The pilot burner 1902 may be oriented to cause contact of the pilot flame with the combustible main fuel and air mixture.

According to an embodiment, the one or more main fuel nozzles 1908 may be configured to output fuel in co-flow with the air, and/or may form a fuel dump plane at the proximal location. The proximal location may be coincident with or near a floor 1928 of the furnace 1904.

The burner system 1900 may further include a distal flame holder 1930 disposed at a distal position along the fuel and combustion air flow axis, similar in position to the one or more main fuel nozzles 1908 as that of the pilot burner 1902. In an embodiment, the distal flame holder 1930 may be fabricated with metal, or entirely from metal, or may consist essentially of metal. In other embodiments, the distal flame holder 1930 may include other materials such as ceramic, refractory materials, and the like.

The burner system 1900 may further include a support structure exemplified by support legs 1934 supporting the distal flame holder 1930 in the furnace 1904, and a mixing tube 1940, shown in FIG. 19 in dashed lines.

The distal flame holder 1930 as illustrated in FIG. 19 exemplifies a gutter-type flame holder made of metal or other material. A gutter-type flame holder refers generally to an elongate bluff body. A V-gutter generally refers to a V-shaped elongate bluff body that is oriented with the open side of the V away from a direction of impinging flow. As used herein, it will be understood that the terms gutter, V-gutter, gutter-type flame holder, and elongate bluff body shall be considered synonymous, unless context indicates otherwise.

Embodiments of the distal flame holder 1930 may include a solid refractory body, a solid ceramic body, and/or a perforated or porous ceramic such as a reticulated ceramic similar to the distal flame holder 102 described above with respect to FIG. 7. The distal flame holder 1930 may be configured to support a combustion reaction of the fuel and combustion air upstream, downstream, and within the distal flame holder 1930. This embodiment of the distal flame holder 1930 may include an input face (e.g., 712); an output face (e.g., 714); and a plurality of perforations (e.g., 710) extending between the input face and the output face. The perforations may be formed as passages between the reticulated fibers.

FIG. 20 illustrates a method 2000 associated with operating a burner system, including a step 2002 of providing a pre-mixture of pilot fuel and oxidant to a pilot burner. FIG. 20 illustrates a particular embodiment where a main combustion reaction is ignited by a distal flame holder kept at or above an auto-ignition temperature of the main fuel and combustion air. Step 32004 includes providing heat to a distal flame holder from a pilot flame supported by the pilot burner, the pilot flame being fueled by a pilot fuel, the distal flame holder and the pilot burner being disposed in a furnace and in proximity to one another, the pilot burner disposed near the distal flame holder. The method 2000 may also include providing a distance between the pilot burner and the distal flame holder smaller than a distance between the pilot burner and the one or more main fuel nozzles. It may also include a step 2006 of introducing main fuel and air to the distal flame holder and a step 2008 of holding at least a portion of a combustion reaction of the mixed main fuel and air with the distal flame holder while the pilot burner continues to support the pilot flame.

FIG. 21A illustrates the method of step 2002 associated with operating a burner system, according to an embodiment. In step 2102, pilot fuel is supplied to a pilot premix chamber via a fuel fitting (not via a premix fuel nozzle). Step 2104 includes supplying pilot oxidant (such as pilot combustion air) to the pilot pre-mix chamber via a pre-mix oxidant channel. Step 2106 includes mixing the pilot fuel and the oxidant in the pilot pre-mix chamber. Step 2108 includes outputting the pre-mixture through a pilot burner premix nozzle. Step 2110 includes igniting the pre-mixture via pilot igniter to ignite the pilot flame. The step 2110 may further include a step wherein the pilot igniter 1926 (see FIG. 24) provides a spark, and/or a step wherein a pilot igniter (not shown) provides a hot surface heated to at least an auto-ignition temperature of the pre-mixture.

FIG. 21B illustrates a method of step 2004 associated with operating a burner system, according to an embodiment that includes a step 2105 of controlling a pre-mixture rate of flow, a step 2111 of igniting the pre-mixture to provide the pilot flame PF, and a step 2112 of igniting a mixture of the main fuel and air at the pilot burner 1902 with the pilot flame PF, to obtain the main flame MF.

Referring again to FIG. 20, the method for operating a burner system may include a step 2005 of measuring a temperature of the distal flame holder, and a step 2010 of reducing a pilot fuel rate of flow to reduce a size of the pilot flame when the temperature of the distal flame holder is at or above a predetermined threshold, wherein reducing the size of the pilot flame relative to the size of the combustion reaction of the mixed main fuel and air causes a reduction of emissions of oxides of nitrogen. FIG. 20 may be understood by substituting a step 2009, “determine that a temperature of the distal flame holder is at or above a predetermined threshold” for the specific wording in the figure.

FIG. 22 shows a flow chart illustrating a method associated with step 2006 of FIG. 20, for operating a burner, according to an embodiment. Step 2006 of FIG. 20 includes introducing mixed main fuel and air to the distal flame holder, which may include: a step 2202 of introducing, at a proximal end of the mixing tube (e.g., 1940), the main fuel via the one or more main fuel nozzles and the air; a step 2204 of educing a flue gas into the proximal end of the mixing tube 1940; and a step 2206 of mixing the main fuel and the air in the mixing tube 1940. In the step 2206, the proximal end of the mixing tube 1940 may be disposed proximate to the one or more main fuel nozzles, and a distal end of the mixing tube 1940 being disposed proximate to the distal flame holder. The mixing tube 1940 is open from the proximal end to the distal end.

FIG. 23A illustrates a method 2300 associated with operating a burner system, according to an embodiment. In step 2302, a pilot flame is held across at least a portion of a width of a furnace volume at a position distal from a furnace floor. Step 2314 includes providing combustion air to the furnace volume from a location near the furnace floor; and 2316, outputting a high pressure main fuel jet from each of one or more main fuel nozzles at one or more locations near the furnace floor. Step 2318 includes mixing the main fuel with the combustion air while the main fuel and combustion air travel from the locations near the furnace floor to the distal position, followed by a step 2320, including combusting the main fuel to produce a main flame by exposing the mixed main fuel and air to the pilot flame.

FIG. 23B illustrates a method associated with operating a burner system, according to an embodiment. The method of FIG. 23B may include an operation 2304, supplying a pilot fuel to a pilot pre-mix chamber via a fitting Operation 2306 may include supplying an oxidant to the pilot pre-mix chamber via a pre-mix oxidant channel. Operation 2308 may include mixing the pilot fuel and the oxidant in the pilot pre-mix chamber.

Operation 2310 in FIG. 23B includes outputting the pre-mixture of pilot fuel and oxidant from a pilot fuel nozzle. In operation 2312 the pre-mixture is ignited by a pilot igniter to provide a pre-mixed flame. Operation 2322 includes holding the main flame, resulting from said combusting the main fuel, at a stable position with a distal flame holder disposed more distal from the furnace floor than the pilot flame. In addition, the step 2302 may include an operation 2324, detecting the combustion of the main fuel at the distal flame holder using an electrocapacitive sensor. The electrocapacitive sensor may be configured to output sensor signals to a controller.

Some of the features of FIG. 19 are also illustrated in FIG. 24 and denoted with similar reference numbers. FIG. 24 illustrates a combustion system 2400, according to an embodiment. The combustion system 2400 may include an oxidant source 2402 configured to output an oxidant into a furnace volume 2404 and a pilot burner 1902 configured to pre-mix pilot fuel and oxidant to support a pilot pre-mixed flame PF during at least one operation state. The combustion system 2400 may also include a main fuel nozzle 1908 configured to output a main fuel into the furnace volume 2404 from a proximal position during a standard operating state at least after the preheating state is complete and a distal flame holder 1930 positioned in the furnace volume 2404 to be preheated by the pilot pre-mixed flame during the preheating state and to hold a combustion reaction of the main fuel and oxidant adjacent to the distal flame holder 1930 during the standard operating state. In addition, the combustion system 2400 may also include: one or more combustion sensor(s) 2406 configured to sense a condition of the distal flame holder 1930 and to generate a sensor signal indicative of the condition of the distal flame holder 1930; an actuator 2408 configured to adjust a flow of the main fuel from the main fuel nozzle 1908; an actuator 2410 to adjust a flow of at least one of the pilot fuel and the oxidant to the pilot burner; and an actuator 2412 to adjust a flow of the oxidant from the oxidant source. A controller 2414 may be communicatively coupled to the one or more actuators 2408, 2410, 2412 and the combustion sensor 2406, and the controller 2414 may be configured to receive the sensor signal from the combustion sensor 2406 and to control the one or more actuators 2408, 2410, 2412 to adjust the flow of the pilot fuel, the main fuel, and/or the oxidant responsive to the sensor signal and in accordance with software instructions stored in a non-transitory computer readable medium coupled to the controller.

The controller 2414 may also be coupled to a pilot flame sensor 2416 that may be configured to sense a condition of the pilot pre-mixed flame PF and to output to the controller 2414 a sensor signal indicative of the condition of the pilot pre-mixed flame PF. The combustion sensor 2406 may, in an embodiment, include the pilot flame sensor 2416. The pilot flame sensor 2416 may include an electrocapacitive sensor (e.g., 1320 described above with respect to FIGS. 13A-B).

In an embodiment, the controller 2414 may be configured to control one or more of the actuators 2408, 2410, 2412 to cause the igniter 1926 to ignite the pilot pre-mixed flame if the pilot flame sensor 2416 indicates that the pilot pre-mixed flame is not present and all safety interlocks are satisfied. The pilot flame sensor 2416 may include an electro-resistive sensor and/or a tomographic sensor. The controller 2414 may be configured to adjust a size of the pilot pre-mixed flame in response to the sensor signals from at least the combustion sensor 2406 by controlling one or more of the actuators 2408, 2410, 2412 to adjust the flow of the pilot fuel and/or the oxidant. The combustion sensor 2406 may be further configured to detect the combustion reaction at the distal flame holder 1930 and to output sensor signals to the controller 2414 responsive to a detected state of the combustion reaction. The combustion sensor(s) 2406 may include an electrocapacitive sensor.

In an embodiment, the electrocapacitive sensor may include a first set of electrodes positioned laterally around the distal flame holder (see FIG. 13B); this is exemplified in FIG. 24 by the electrode(s) 2406 including portions or electrodes on either side of the distal flame holder 1930. The electrocapacitive sensor may be configured to sense a parameter in a vicinity of the distal flame holder, and optionally may further include a second set of electrodes positioned upstream from the distal flame holder as shown in FIG. 24 and may be configured to sense a parameter upstream from the distal flame holder, and the controller 2414 may sense the combustion reaction by comparing a parameter sensed by the first set of electrodes to a parameter sensed by the second set of electrodes. In an embodiment, the first set of electrodes are part of the combustion sensor. In an embodiment, the second set of electrodes positioned upstream from the distal flame holder 1930 may be configured to detect flashback by sensing a parameter upstream from the distal flame holder 1930. The electrocapacitive sensor may include a plurality of electrodes positioned laterally around the distal flame holder as shown in FIG. 24 (see also FIGS. 13A-B).

In an embodiment, the plurality of electrodes 2406 may include one or more first pairs of electrodes separated from each other by the distal flame holder and disposed opposite each other in a first orientation substantially perpendicular to a primary direction of a flow of the main fuel toward the distal flame holder, as shown in FIG. 24. In such a configuration, the plurality of electrodes may include one or more second pairs of electrodes 2406 separated from each other by the distal flame holder 1930 and disposed opposite each other in a second orientation substantially perpendicular to both the first orientation and the primary direction of the flow of the main fuel, and/or the axis 1006.

Now referring again to FIG. 19, this figure also illustrates a distal assembly for a burner. This assembly may include: that portion of the mixing tube 1940 defining an outflow end thereof (in the figure, an upper end); a support structure adjacent to the outflow end of the mixing tube 1940; one or more flow disruptors disposed in a fluid flow channel aligned with the outflow end of the mixing tube 1940 configured to cause one or more low flow velocity zones in the fluid flow channel (in the figure, the distal flame holder 1930 is exemplified by V-gutter flame holders, which have this effect and act a flow disruptors); and the pilot burner 1902, which may be disposed to cause ignition of a fuel and combustion air mixture coincident with or upstream of the one or more low flow velocity zones. In this embodiment, the pilot burner may be configured to maintain a pilot flame PF of sufficient output power to cause the ignited fuel and combustion air held by the one or more flow disruptors 1930 to ignite substantially all fuel delivered by the fluid flow channel through the portion of the mixing tube 1940 defining the outflow end thereof.

In FIG. 19, the support structure 1934 is shown on the left side of the figure to be mechanically coupled to the mixing tube 1940, but it may also or alternatively extend to the floor 1928 of a furnace volume 1904 (labelled in FIG. 24). In this embodiment, the pilot pre-mix chamber 1910 may be operable to cause substantially complete mixing of the flow of pilot air and pilot fuel. The pre-mix fuel nozzle 1912 may be coupled to the pre-mix chamber 1910 and be configured to support a momentum-regime flame aimed into a region coincident with an intended combustion position, as shown in FIG. 1. One or more flow disruptors may define a low velocity and/or low pressure zone in the fluid flow channel that act to hold and stabilize the location of a main combustion reaction.

The inventors have observed that when using a diffusion pilot it is beneficial to spread a flame across an area of fuel and/or oxidant flow. For example, a flame holder may span the area of fuel/oxidant flow. In contrast, in an embodiment employing a pre-mix pilot the flame momentum may be sufficient to distribute the flame without employing a flame-spreading structure. The flame holder 1930 described and illustrated for embodiments employing a pre-mix pilot may constitute or incorporate one or more flow disruptors. As used herein, a flow disruptor is used as a generic term for an object that causes a disruption in fluid flow. The flow disruption caused by a flow disruptor generally includes vortices, eddies, low pressure regions compared to the pressure of overall flow, and/or low velocity regions compared to overall flow velocity. A flow disruptor creates a condition for mass, momentum, and heat mixing or recycling into a larger flow. In some embodiments, a flow disruptor creates a low velocity condition conducive to protection of an ignited flame, such as a pilot flame, from a relatively high flow rate of a fuel and air mixture in adjacent regions, so as to prevent blow-off of the ignited flame. Types of flow disruptors referenced herein include bluff bodies, perforated or porous bodies, swirlers, V-gutters, and the like. The flow disruptor, may perform a flame holding function in a burner system. Accordingly, the term flow disruptor may be used interchangeably with the term flame holder. In other embodiments, a flow disruptor may be displaced relatively distant from a flame holding region, such as in the case of an upstream swirler configured to cause a gas expansion at an outlet end of a mixing tube to produce a low pressure region that functions to hold a combustion reaction.

In an embodiment, as shown in FIG. 25, the one or more flow disruptors 1930 may define boundaries of a partially-bounded box 2500, the partially-bounded box 2500 being defined as a low fluid flow velocity volume bounded by at least one flow disruptor 2502 defining a perimeter wall disposed parallel to the fluid flow channel 2510, and at least a second flow disruptor 2504 forming a wall transverse to the fluid flow channel and downstream from at least a portion of the flow disruptor 2502 defining the perimeter wall. The partially-bounded box 2500 may include a third flow disruptor 2506 forming a wall transverse to the fluid flow channel and upstream from at least a portion of the flow disruptor 2502 defining the perimeter wall. The pilot burner 1902 may be disposed to expose a combustible mixture of fuel and combustion air disposed within the low fluid flow velocity volume bounded by the at least first and second flow disruptors.

In an embodiment, the pilot burner 1902 may include a pre-mixed burner, wherein the pilot burner 1902 may be disposed to shoot a momentum dominated regime pre-mix flame PF into the low fluid flow velocity volume.

FIG. 26 illustrates, in an embodiment, that one or more flow disruptors 1930 may include two or more V-gutters disposed along intersecting diameters or diagonals of the portion of the mixing tube 1940 defining the outflow end, and may further include a swirler 2604 disposed on a center shaft 2602 disposed parallel to an axis of the fluid flow channel defined by the portion of the mixing tube 1940 defining the outflow end, which may or may not coincide with and/or be parallel to axis 1906. A swirler may be disposed upstream from the outflow end of the mixing tube 1940 and/or may be disposed on a wall defining the mixing tube 1940, as illustrated by swirler blade 2606.

In an embodiment, the V-gutter 1930 may define a distal pilot burner.

In an embodiment wherein the fluid flow disruptor(s) includes a V-gutter, the V-gutter may be disposed on the center shaft 2602 disposed parallel to an axis of the fluid flow channel defined by the portion of the mixing tube 1940 defining the outflow end. A V-gutter 1932 may be additionally or alternatively disposed on a perimeter surface aligned with edges of the fluid flow channel downstream from the mixing tube 1940. A flow disruptor may include a first metal bluff body 1930 disposed on the center shaft 2602 parallel to an axis of the fluid flow channel. A second metal bluff body 1932 may be disposed on a perimeter surface aligned with edges of the fluid flow channel downstream from the mixing tube 1940. A metal bluff body may be configured as flat strip, a square tube, a round tube, a U-shaped form, a V-shaped form, or other shape configured to create a low velocity and low-pressure region in the fluid flow.

In an embodiment wherein the center shaft 2602 is hollow, fuel nozzles may receive fuel flow through the center shaft. The V-gutter may define a plurality of fuel nozzles operatively coupled to the low fluid flow velocity zone(s).

While various aspects and embodiments have been disclosed herein, other aspects and embodiments are contemplated. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims. 

1. A burner system, comprising: a pilot burner disposed in a furnace at a distal position along a flow axis of a main fuel and combustion air; and one or more main fuel nozzles disposed at a proximal position along the flow axis and configured to output a main fuel; wherein the pilot burner is configured to support a pilot flame; and wherein the one or more main fuel nozzles are configured to support a main flame in contact with the pilot flame; wherein the pilot burner is disposed to cause the main fuel and combustion air to be ignited by the pilot flame.
 2. The burner system of claim 1, wherein the main flame comprises a flame having a heat output of at least 10 times the heat output of the pilot flame when the burner system is operating at a rated heat output.
 3. The burner system of claim 2, wherein operating at the rated heat output corresponds to operating in a steady state standard operating mode.
 4. The burner system of claim 2, wherein the main flame comprises a flame having a heat output of at least 20 times the heat output of the pilot flame when the burner system is operating at a rated heat output.
 5. The burner system of claim 1, further comprising a stack operatively coupled to the burner system, wherein the burner system has a NOx output of about twenty parts per million or less, adjusted to 3% excess O₂ at the stack.
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 8. The burner system of claim 1, wherein the pilot burner defines a plurality of fuel orifices having a sufficiently large collective area to collectively support a low momentum pilot flame.
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 12. The burner system of claim 1, wherein the pilot burner comprises a fuel manifold having a plurality of segments joined together, each segment having a plurality of fuel orifices configured to pass fuel from inside the fuel manifold to a furnace combustion volume.
 13. The burner system of claim 12, wherein the plurality of segments are formed as respective tubes configured to freely pass the fuel delivered from a fuel pipe into the fuel manifold.
 14. The burner system of claim 13, wherein at least a portion of the tubes is arranged as spokes radiating from a center disposed substantially at a centerline along the axis.
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 20. The burner system of claim 1, wherein the pilot burner supports a diffusion flame at the distal location at least 100 main fuel nozzle diameters from the floor of the furnace.
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 22. The burner system of claim 1, wherein the pilot burner includes at least one tube disposed transverse to the fuel and combustion air flow axis.
 23. The burner system of claim 22, further comprising: one or more sections of reticulated ceramic disposed superjacent to the at least one tube.
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 26. The burner system of claim 1, further comprising: a distal flame holder disposed at a third position along the fuel and combustion air flow axis, more distal from the main fuel nozzles than the pilot burner.
 27. The burner system of claim 26, wherein the distal flame holder comprises a perforated flame holder.
 28. The burner system of claim 1, wherein the pilot burner is a pre-mix burner configured to support the pilot flame using a pre-mixture of a pilot fuel and an oxidant.
 29. The burner system of claim 28, wherein the pilot burner includes: a pilot pre-mix chamber; a pilot fuel line fitting configured to output the pilot fuel into the pilot pre-mix chamber; a pilot oxidant channel configured to output oxidant into the pilot pre-mix chamber; and a pilot pre-mixture nozzle arranged to receive the pre-mixture of pilot fuel and oxidant from the pilot pre-mix chamber and output the pre-mixture of pilot fuel and oxidant into the furnace to support the pilot flame; wherein the pilot pre-mix chamber, the pilot fuel line fitting, and the pilot oxidant channel are arranged to cause mixing of the pilot oxidant with the pilot fuel in the pilot pre-mix chamber to produce the pre-mixture of pilot fuel and oxidant
 30. (canceled)
 31. The burner system according to claim 29, wherein the pilot burner further includes a flame arrestor disposed to cause the pre-mixture of pilot fuel and oxidant to flow through the flame arrestor as the pre-mixture of pilot fuel and oxidant flows from the pilot pre-mix chamber through the pilot pre-mixture nozzle.
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 34. The burner system according to claim 29, wherein the pilot pre-mix chamber is disposed closer to the proximal position than to the distal position; wherein the pilot pre-mix chamber comprises a pre-mixture pipe arranged to deliver the pre-mixture of the pilot fuel and oxidant from the pilot fuel line and pilot oxidant channel to a pilot burner distal assembly disposed adjacent to the distal position; and wherein the pilot burner distal assembly includes a flame arrestor arranged to pass the pre-mixture of the pilot fuel and oxidant from the pre-mixture pipe to the pilot pre-mixture nozzle and to prevent a flash-back of combustion from the pilot pre-mixture nozzle into the pilot pre-mixture pipe.
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 37. The burner system according to claim 34, wherein the pilot burner distal assembly includes the pilot pre-mixture nozzle; and wherein the pre-mixture pipe is configured to transmit the pilot fuel, the oxidant, and the pre-mixture thereof at a sufficiently high flow rate to cause a flow velocity to exceed a flame speed of the fuel and oxidant pre-mixture to prevent a flash-back of combustion into or through the pre-mixture pipe.
 38. The burner system according to claim 28, further comprising: a pilot igniter configured to ignite the pre-mixture of the pilot fuel and oxidant after the pre-mixture of the pilot fuel and oxidant is emitted from the pilot pre-mixture nozzle.
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 48. The burner system of claim 27, wherein the perforated flame holder comprises perforations formed as passages between the reticulated fibers or in a reticulated ceramic foam.
 49. A method for operating a burner system, comprising: providing heat to a distal flame holder from a pilot flame supported by a pilot burner, the pilot flame being fueled by a pilot fuel, the distal flame holder and the pilot burner being disposed in a furnace and in proximity to one another, the pilot burner disposed between the distal flame holder and one or more main fuel nozzles, a distance between the pilot burner and the distal flame holder being smaller than a distance between the pilot burner and the one or more main fuel nozzles; introducing mixed main fuel and air to the distal flame holder; and holding at least a portion of a combustion reaction of the mixed main fuel and air with the distal flame holder while the pilot burner continues to support the pilot flame.
 50. (canceled)
 51. The method of claim 49, further comprising: measuring a temperature of the distal flame holder; and when the temperature of the distal flame holder is at or above a predetermined threshold, reducing a pilot fuel rate of flow to reduce a size of the pilot flame; wherein reducing the size of the pilot flame relative to the size of the combustion reaction of the mixed main fuel and air causes a reduction of emissions of oxides of nitrogen.
 52. The method of claim 49, wherein introducing mixed main fuel and air to the distal flame holder includes introducing, at a proximal end of a mixing tube, the main fuel via the one or more main fuel nozzles and the air; wherein the proximal end of the mixing tube is disposed proximate to the one or more main fuel nozzles, and a distal end of the mixing tube is disposed proximate to the distal flame holder, the mixing tube being open from the proximal end to the distal end.
 53. The method of claim 52, further comprising: educing a flue gas into the proximal end of the mixing tube.
 54. The method of claim 52, wherein the pilot burner is disposed between the distal flame holder and the distal end of the mixing tube.
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 64. The method of claim 51, further comprising: detecting the combusting of the main fuel at the distal flame holder using an electrocapacitive sensor, the electrocapacitive sensor configured to output sensor signals to a controller.
 65. The burner system according to claim 1, further comprising: a distal flame holder positioned in the furnace in a position to be preheated by the pilot flame during a preheating state and to hold a combustion reaction of the main fuel and oxidant adjacent to the distal flame holder during a standard operating state; a pilot flame sensor configured to sense a condition of the pilot flame and to output a sensor signal indicative of the condition of the pilot flame; a combustion sensor configured to sense a condition of the distal flame holder and to generate a sensor signal indicative of the condition of the distal flame holder; one or more actuators configured to adjust a flow of the main fuel from the one or more main fuel nozzles, to adjust a flow of pilot fuel to the pilot burner, and to adjust a flow of oxidant from an oxidant source; and a controller communicatively coupled to the actuators and the combustion sensor, the controller being configured to receive the sensor signal from the combustion sensor and to control the actuators to adjust the flow of the pilot fuel, the main fuel, and the oxidant responsive to the sensor signal and in accordance with software instructions stored in a non-transitory computer readable medium coupled to the controller.
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 68. The burner system according to claim 65, wherein the pilot flame sensor includes at least one of an electrocapacitive sensor, an electro resistive sensor, and a tomographic sensor.
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 73. The burner system according to claim 65, wherein the controller is configured to adjust a size of the pilot flame in response to the sensor signals from at least the combustion sensor by controlling one or more of the actuators to adjust the flow of the pilot fuel or the oxidant.
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 77. The burner system according to claim 68, wherein the electrocapacitive sensor includes: a first set of electrodes positioned laterally around the distal flame holder and configured to sense a parameter in a vicinity of the distal flame holder.
 78. The combustion system of claim 77, wherein the electrocapacitive sensor includes: a second set of electrodes positioned upstream from the distal flame holder and configured to sense a parameter upstream from the distal flame holder.
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 82. The burner system according to claim 68, wherein the electrocapacitive sensor includes a plurality of electrodes positioned laterally around the distal flame holder, and wherein the plurality of electrodes include one or more pairs of electrodes each separated from each other by the distal flame holder.
 83. (canceled)
 84. The burner system according to claim 82, wherein the electrocapacitive sensor generates electrocapacitive tomography images based on a capacitance between the one or more pairs of electrodes.
 85. The burner system according to claim 82, wherein at least one of the pairs of electrodes is separated by the distal flame holder and disposed opposite each other in a first orientation substantially perpendicular to a primary direction of a flow of the main fuel toward the distal flame holder.
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 90. A low emissions modular burner system, comprising: one or more burner modules, each burner module including: a main fuel source, separately valved from all other fuel sources, configured to selectively deliver a main fuel stream for dilution by a flow of combustion air, a main fuel igniter configured to cause ignition of the main fuel stream emitted from the main fuel source, a distal flame holder, separated from the main fuel source and the main fuel igniter by respective non-zero distances, the distal flame holder being configured to hold a combustion reaction supported by the main fuel stream when the distal flame holder is at or above a predetermined temperature, and a pre-heating apparatus configured to pre-heat the distal flame holder to the predetermined temperature; a common combustion air source configured to provide combustion air to each of the plurality of burner modules; and a wall encircling all of the one or more burner modules, the wall being configured to laterally contain combustion fluids corresponding to the one or more burner modules.
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 96. The low emissions modular burner system of claim 90, further comprising: one or more separate main fuel valves, each including a separate main fuel valve actuator configured to operate responsive to receiving control signals; and a control system configured to output respective control signals to each of the separate valve actuators; wherein the control system further comprises: an interface between the control system and an input channel, wherein the interface is configured to receive a signal corresponding to a burner capacity requirement; one or more burner module sensor inputs, each of the one or more burner module sensor inputs being configured to receive a signal corresponding to a burner module status wherein the burner module status is provided by sensor hardware; a microcontroller, a computer readable memory, and a module sequencer configured to select a subset of the one or more burner modules for ignition; and a respective one or more main fuel valve driver outputs each operatively coupled to one of the separate main fuel valve actuators.
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 100. The low emissions modular burner system of claim 96, wherein the microcontroller is configured to read and execute computer executable instructions, supported by a non-transitory computer readable memory, to: receive capacity input data corresponding to the burner capacity requirement signal; read module status sensor data from one or more sensors corresponding to at least one burner module to verify that a selected one or more of the burner modules is ready for firing; select the subset of the one or more burner modules for firing; and drive at least one of the separate main fuel valve actuators corresponding to the selected subset of the one or more burner modules to open so as to provide fuel to a combustion reaction supported by the subset of the one or more burner modules.
 101. (canceled)
 102. The low emissions modular burner system of claim 90, wherein the pre-heating apparatus comprises: a pilot fuel source configured to provide a pilot fuel, a pilot fuel igniter configured to ignite a flow of the pilot fuel, and a distal pilot configured to hold a pilot flame supported by the pilot fuel, a pilot fuel source flow rate being selected to provide a pilot flame sized to raise the distal flame holder temperature to the pre-determined temperature.
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 115. The low emissions modular burner system of claim 90, wherein at least one of the burner modules is configured to be freestanding, supported only by a coupling at a combustion air inlet.
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